Refactor voice selection menu to its own component using event-based
pattern for voice selection and preview.

This is in preparation of future additions where we'll be adding more
complex logic for voice selection. Having a voice-selection component
keeps read-anything-toolbar TS code cleaner.

The event-based pattern allows us to decouple the voice-selection
component from the read-anything-app component, eliminating the need
for the child component to directly call its parent's functions (via
`this.contentPage.`). Without this, it would have been messy to break
voice selection to its own component, because it would've needed to
call read-anything-toolbar's methods which calls read-anything-app's
methods.

Also, the event based pattern allows us to move away from passing
callbacks to the HTML (via MenuStateItem) and later invoking that
callback from TS.


Bug: 1474951
Change-Id: Ifcd29d5ac341f4b270bb41bf78a79fdedc8a7add
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5245249
Reviewed-by: Eitan Goldberger <eitang@google.com>
Reviewed-by: Lauren Winston <lwinston@google.com>
Commit-Queue: Eitan Goldberger <eitang@google.com>
Cr-Commit-Position: refs/heads/main@{#1254088}