Add keyboard accessibility

[vahissan]

Description

• Added ability to navigate the calendar using the TAB key
• Added ability to close the calendar by pressing the ESC key when focused on the calendar or the input field
• Added ability to open the calendar by pressing the ArrowDown key (useful when it was closed by ESC key)

Motivation and Context

Fixes #415 (at least partially)
I was using the library and would like to have the ability to operate the date picker without a mouse. I'm sure many others would like it too.

Checklist

[ x ] I have not included any built dist files (us maintainers do that prior to a new release)
[ x ] I have added tests covering my changes
[ x ] All new and existing tests pass
[ ] My changes required the documentation to be updated
  [ ] I have updated the documentation accordingly
  [ ] I have updated the TypeScript 1.8 type definitions accordingly
  [ ] I have updated the TypeScript 2.0+ type definitions accordingly

[vahissan]

These are the things from the original issue which I didn't add in this PR:

• aria-label attributes for the td cells: I don't think it's required because the screen readers pickup the text inside the td elements without issue.
• arrow key navigation within the calendar: I think it would impact the performance. Also, I wanted to add the basic functionality first, and maybe improve it later.

[vahissan]

CI build failed due to these reasons:

1. There's a peer dependency check failing on npm install due to npm v7's breaking changes.
2. Unit tests failed because they depend on the built dist files.

[arqex]

Hey @vahissan !

Thanks for this great PR, I need to find some time to review it :)

If you know what's failing with the v7 of npm, can create a different PR with the fix and we merge it before this one?

[vahissan]

Thanks @arqex !

Sure, I'll take a look. Adding --legacy-peer-deps to npm install in CI is a workaround if you're interested.

EDIT: created PR #758

[vahissan]

CI build failed due to these reasons:

1. There's a peer dependency check failing on npm install due to npm v7's breaking changes.
2. Unit tests failed because they depend on the built dist files.

Reason 1 is resolved. But CI still fails due to reason 2. Looks like CI does not create a new build before running unit tests.

[vahissan]

Hey @arqex !

I know you must be busy. Just wanted to send a reminder in case you forgot.