Migrate to Tailwind plugin, use @low and @high priorities, new docs, and more

This large update to a17t migrates it from a traditional CSS library to a Tailwind CSS plugin! This is an incredibly exciting update, which will make a17t an even more compelling way to build interfaces.

With such a large update, though, come some fundamental changes.

Major Changes

• There are only two priorities: @low and @high. I made this change because the middle priority — @normal — often felt forced. In many cases, the @low priority variant also didn't make much sense. In this update, !normal is now @low, and !high is now @high.
• You might have noticed that priorities now use the "at" symbol as the prefix rather than an exclamation point. I changed the prefix for the priority classes because Tailwind's default CSS purge system wasn't detecting classes with the exclamation point prefix. Using an at symbol just makes everything much more reliable. I wish we didn't have to make this change, but we do. (Fortunately, migration is as simple as a find and replace.)
• a17t no longer can be distributed as a simple CSS file; rather, it needs to be installed as a Tailwind CSS plugin. I'm confident this is the right direction for the project, but I also acknowledge that this will inconvenience some existing users. I'm sorry! But I promise using Tailwind is worth it.
• Buttons look slightly different now. (They look better.)
• Cards function slightly differently now. Specifically, card on its own is a neutral white card, while card ~neutral is a gray card, and card ~neutral @high is a dark gray card. (This change preserves the old plain white card while also easily supporting the new @low and @high priorities.
• CSS variable customization is no longer supported. While this was one of a17t's original differentiators, in retrospect it was misguided. There is already a much better way to customize the toolkit: using tailwind.config.js! And since a17t now reads important values like spacing, colors, and border radii from the Tailwind config, you only need to apply customizations once (rather than twice, once for a17t and once for Tailwind).

Minor Changes

• .kbd now has a dark background with white text, rather than the inverse. It was always meant to be white text on a dark background; I'm not sure when that changed, but it was a regression.
• The documentation now has a new landing page.
• The documentation no longer has an off-white background.
• The documentation has more examples.
• Inputs and textareas were removed from the docs, because they are redundant to fields. Going forward, I recommend just using the .field selector for these elements. To preserve backwards compatibility, the .input and .textarea selectors will still work — like before, they are identical to .field. (That is to say that inputs, textareas, and fields were all merged into one.)

What Didn't Change

• a17t is still as atomic and dynamic as it ever was!

More changes

• Bump kramdown from 2.3.0 to 2.3.1 in /docs by @dependabot in #12
• Bump y18n from 4.0.0 to 4.0.1 by @dependabot in #13
• Bump rexml from 3.2.4 to 3.2.5 in /docs by @dependabot in #14
• Bump lodash from 4.17.20 to 4.17.21 by @dependabot in #15
• Bump postcss from 8.2.2 to 8.2.10 by @dependabot in #16
• Bump browserslist from 4.11.0 to 4.16.6 by @dependabot in #18
• Bump glob-parent from 5.1.1 to 5.1.2 by @dependabot in #19
• Bump addressable from 2.7.0 to 2.8.0 in /docs by @dependabot in #21
• Migrate to Tailwind plugin, use @low and @high priorities, new docs, and more by @milesmcc in #22

Full Changelog: v0.5.1...v0.10.1

No more dependencies

11ec019 - R. Miles McCain - Bump version
57c33bd - R. Miles McCain - Emphasize no dependencies
ba11c3e - R. Miles McCain - Update dependencies

Legibility, theming, and documentation improvements

2e011b3 - R. Miles McCain - Bump version to 0.5.0
a766058 - R. Miles McCain - Rebuild
3ef1eaf - R. Miles McCain - Use ease-in-out for button animations (not linear)
4a9a64f - R. Miles McCain - Make field placeholders more legible
8d8b410 - R. Miles McCain - Remove ugly border from aside
c7cfd94 - R. Miles McCain - Fix kbd theming
78e7bb3 - R. Miles McCain - Import more weights of Inter in documentation
dececa0 - R. Miles McCain - Use semantic buttons in button documentation
2621c9d - R. Miles McCain - Set default font weight
65130bd - R. Miles McCain - Integrate new themes into interface
e16ea20 - R. Miles McCain - Add Watson theme
539fbf6 - R. Miles McCain - Improve default dark theme
3dc9820 - R. Miles McCain - Add mono theme
73891a2 - R. Miles McCain - Remove extra space in index
1b92289 - R. Miles McCain - Add 24h expiry to doc themes

Expanded colors and themes

9e77e1e - R. Miles McCain - Bump version
2ce3c1a - R. Miles McCain - Update themes, color system
c7f3c85 - R. Miles McCain - Adjust colors

Upgrade to Tailwind CSS 2.0

9194407 - R. Miles McCain - Bump version to v0.3.0
26355d1 - R. Miles McCain - Upgrade to Tailwind 2.0
caee558 - R. Miles McCain - Merge pull request #11 from weaversam8/patch-2
233f75c - Sam Weaver - Update paragraph to have .content class
5c2b8f6 - R. Miles McCain - Merge pull request #10 from weaversam8/patch-1
8e1237a - Sam Weaver - Add underline to link
27a57c3 - Sam Weaver - Change code example to safelistPatterns instead of whitelistPatterns
f41f0ac - Sam Weaver - Update broken link to PurgeCSS whitelisting
e1ff568 - R. Miles McCain - Add documentation favicon
11e1587 - R. Miles McCain - Update documentation dependencies
3553d1e - R. Miles McCain - Update dependencies
86ce22a - R. Miles McCain - Add flex-shrink-0 to image card on homepage
ef2ad8d - R. Miles McCain - Fix pre code background color

Documentation improvements & dependency updates

e1ff568 - R. Miles McCain - Add documentation favicon
11e1587 - R. Miles McCain - Update documentation dependencies
3553d1e - R. Miles McCain - Update dependencies
86ce22a - R. Miles McCain - Add flex-shrink-0 to image card on homepage

Fix pre code background color (hot fix)

ef2ad8d - R. Miles McCain - Fix pre code background color

Content syntax highlighting improvements

6dc59da - R. Miles McCain - Bump version
4618dd9 - R. Miles McCain - Fix syntax highlighting in content

General typography improvements

d29bcf9 - R. Miles McCain - Rebuild dist files
0e58de3 - R. Miles McCain - Make content bolds bolder
e914ab0 - R. Miles McCain - Improve code element
c1e62c4 - R. Miles McCain - Add Tailwind documentation
949c8dc - R. Miles McCain - Update dependencies
23b0a2d - R. Miles McCain - Bump version
cd84c28 - R. Miles McCain - Update distribution
66513bc - Igor Barakaiev - Remove --color-title from .content strong

Expand Tailwind CSS options (add group-hover)

v0.2.5

Bump version