Opt into html language features

[Whoops]

This opts into VSCode's built-in HTML language features, allowing for autocomplete and documentation based on HTML tags, attributes, etc.

Addresses #20 and #18.

The extension.js implementation is cribbed directly from Jinja's implementation to address microsoft/vscode#160585 as mentioned in the comment.

[RedCMD]

It is not required to manually activate the html extension await htmlExtension?.activate();
because "extensionDependencies" already activates the extension for you

I don't think a blank extension.js file is required either

[Whoops]

It didn't activate the extension for me, that's how I found the vscode and Jinja issues. When I first tested it with just the package.json changes I had no auto complete or documentation on hover.

[RedCMD]

weird
it works when I add the package code to my non-html extension

[Whoops]

I retested removing "main": "./src/extension.js", from the package.json and it quit working for me until switching to HTML and back as described in microsoft/vscode#160585. I don't have any experience with VS Code extensions, so I can't really guess what might be different, but the discussion in that issue, especially that it sounds like they are considering auto-activating based on htmlLanguageParticipants makes me think there isn't an obvious better way.

[Whoops]

Interestingly, a blank extension.js file DOES work for me... which seems weird. I can only assume that running the activation script is enough to activate the dependencies, but just declaring them isn't? I've dropped the contents of extension.js since it apparently isn't needed after all.

[RedCMD]

nice catch
could prob make a bug report I guess

BTW after adding "main"
a blank "activationEvents": [] is required for publishing

[Whoops]

Added, thanks! I can probably write up a bug report later this evening.