feat: add aria-checked support to gridcell and row

[smhigley]

Closes #1960

Adds aria-checked as a supported state to gridcell, with the same wording around combining aria-checked & aria-selected as on option and treeitem. The aria-checked support also inherits into columnheader and rowheader.

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.

• [x] "author MUST" tests: N/A
• [x] "user agent MUST" tests: N/A
• [ ] Browser implementations (link to issue or commit):
  • WebKit:
  • Gecko:
  • Blink:
• [ ] Does this need AT implementations? Likely not, since this should come from browser mappings
• [x] Related APG Issue/PR: N/A
• [ ] MDN Issue/PR:

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