Update Canvas 2d attribute names

[yiyix]

• At least two implementers are interested (and none opposed):

  • Chrome
  • Firefox: CSS Text Modifiers for Canvas2D #5617 (comment)
  • WebKit: CSS Text Modifiers for Canvas2D #5617 (comment)

• Tests are written and can be reviewed and commented upon at:
  Rename Canvas2D.textLetterSpacing to Canvas2D.letterSpacing web-platform-tests/wpt#29822
  Rename Canvas2D.textWordSpacing to Canvas2D.wordSpacing web-platform-tests/wpt#29821

• Implementation bugs are filed:

  • Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=1233706
  • Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1708327
  • Safari: https://bugs.webkit.org/show_bug.cgi?id=225181

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

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/canvas.html ( diff )

[domenic]

Looks great! Can you restore the PR template though? I think it'd be valuable to:

• Make sure at least one other browser agrees with the change
• Ensure this update has web platform tests
• Double-check that either the original bugs for the feature are still open, or if they are closed because the feature was implemented in its original form, we file new bugs specifically about update