Remove ::select-fallback-button

This patch removes customizable <select>'s fallback <button> element in
the UA shadowroot by replacing it with the same "InnerElement" <div>
which is used for appearance:auto.

Several supporting changes were made in order to implement this:
- Also remove ::select-fallback-button-text
- Add -internal-appearance-auto-base-select() support for the following
  properties:
  - min-block-size
  - min-inline-size
  - padding-block-start
  - padding-block-end
  - padding-inline-start
  - padding-inline-end
  - content in select::after
- Make <select> focusable again when there is no child <button> element
- Remove dependency on popover triggering code in HTMLFormControlElement
  now that we aren't always opening the popover with a <button>

This is in response to feedback here:
w3c/csswg-drafts#10717

This also addresses additional feedback I received privately that author
styles applied to the select element itself weren't applying to the
in-page portion of the element.

Change-Id: I0a833b370babe9a8ab937822a3578d839510a8fc