Define customizable <select>

Define several concepts related to customizable <select>:

- Relax the HTML parser to allow more tags inside <select>
- Update the content model to allow more elements inside <select> and related elements.
- Define rendering and some interactions for base appearance for <select>
- Add the <selectedcontent> HTML element
- Add a UA stylesheet for <select> with base appearance

Closes whatwg#9799. Closes whatwg#10310. Closes whatwg#10317. Closes whatwg#10520. Closes whatwg#10762.

The following pull requests are superseded: closes whatwg#10557; closes whatwg#10586; closes whatwg#10629; closes whatwg#10633; closes whatwg#10670.

Author: josepharhar