Add class as an alias for className

[Psychpsyo]

This adds class as an alias for className to be more consistent and intuitive. This is now possible since 'class' is no longer a globally reserved word in Javascript.

fixes #1310

(still working on the below checklist)

• [ ] At least two implementers are interested (and none opposed):
  • …
  • …
• [x] Tests are written and can be reviewed and commented upon at: https://github.com/web-platform-tests/wpt/pull/48220
• [x] Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/367440980
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1919264
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=279817
• [ ] MDN issue is filed: …
• [x] The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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