Add `class` and `id` mappings

[smockle]

Closes https://github.com/w3c/html-aam/issues/553

Initially filed as https://github.com/w3c/html-aam/pull/559

@aleventhal provided mappings in https://github.com/w3c/html-aam/issues/553. I verified they match up with Chromium’s mappings:

id

• kHtmlId maps to id in ax_platform_node_base.cc;l=1506-1509
• kHtmlId maps to UIA_AutomationIdPropertyId in ax_platform_node_win.cc;l=5254-5264 (via GetAuthorUniqueId())
• kHtmlId maps to AXDOMIdentifier in ax_platform_node_cocoa.mm;l=1594-1601

class

• kClassName maps to class in ax_platform_node_base.cc;l=1492-1497
• kClassName maps to UIA_ClassNamePropertyId in ax_platform_node_win.cc;l=5265-5269
• kClassName maps to AXDOMClassList in ax_platform_node_cocoa.mm;l=1577-1592

Reviewers: Please also check the formatting. I’ve attempted to match existing styles, but those are inconsistent, e.g. “Object attributes” is bold in datetime (but not in draggable); it uses <code> in src (but not in abbr).

Previews

• class
• id

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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: Bug for AXDOMClassList on Webkit
  • Blink: current implementation
• [ ] Does this need AT implementations?
• [ ] Related APG Issue/PR:
• [ ] MDN Issue/PR: