ARIA IDL updates: convert eligible attributes to enumerated, new ARIA IDL guidance and examples

[rahimabdi]

Closes #2281
Closes #2279

• add enumerated attribute definitions for the following attributes including missing value default, invalid value default, keywords/states and canonical keyword:
  • aria-atomic
  • aria-autocomplete
  • aria-busy
  • aria-checked
  • aria-current
  • aria-disabled
  • aria-expanded
  • aria-haspopup
  • aria-hidden
  • aria-invalid
  • aria-live
  • aria-modal
  • aria-multiline
  • aria-multiselectable
  • aria-orientation
  • aria-pressed
  • aria-readonly
  • aria-required
  • aria-selected
  • aria-sort
  • aria-valuemin
  • aria-valuemax
  • aria-valuenow
• Revise "A. Mapping WAI-ARIA Value types to languages" to include enumerated attributes
• Revise "6.2.4 Value" for permissible values (remove all other types except "enumerated"?)
• Update "6.3 ARIA Attributes"
  • Remove obsolete note in "6.3.4 ARIA nullable DOMString Attributes" about ARIA transitioning to non-nullable DOMString?
  • Remote spec guidance around getting/setting to only apply to non-enumerated attributes
  • Provide updated examples for "6.3.4.1 Example Attribute Usage"

Test, Documentation and Implementation tracking

Once this PR has been reviewed and has consensus from the working group, tests should be written and issues should be opened on browsers. Add N/A and check when not applicable.

• "author MUST" tests:
• "user agent MUST" tests:
• Browser implementations (link to issue or commit):
  • WebKit:
  • Gecko:
  • Blink:
• Does this need AT implementations?
• Related APG Issue/PR:
• MDN Issue/PR:

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