Define base appearance for select multiple drop-down

[josepharhar]

This PR allows drop-down select elements with the multiple attribute to have base appearance. It is very similar to base appearance for single-select drop-down select elements.

Fixes #12049

• At least two implementers are interested (and none opposed):
  • Chrome
  • …
• Tests are written and can be reviewed and commented upon at:
  • https://wpt.fyi/results/html/semantics/forms/the-select-element/customizable-select-multiple-popup
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/357649033
  • Gecko: …
  • WebKit: …
• Corresponding HTML AAM & ARIA in HTML issues & PRs:
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

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

[josepharhar]

This needs additional changes to explain that <select multiple size=1> can be either a listbox or a drop-down box in appearance:auto, but is always a drop-down box for appearance:base. I can make those edits in this PR after #11460 is merged.