Update: add mapping tables for customizable select

[scottaohara]

Initial draft of select / button as child of a select mappings.

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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