Refactor FlatTermSelector into a functional component (#33459)

* Refactor FlatTermSelector into a function component

* Combine selectors

* Use shared empty array reference

* Bring back extra checks

* Better names

* Remove search query character limit

Search is debounced now. I don't think there's a need for this limit anymore.
Use also might use tags with two characters, like WP.

* Don't try to create a new term if user doesn't have the correct capability

* Update default query args and set context to view

* Use a11y speak method directly

* Memorize unescaped term names

* Fix typo

* Don't disable FormTokenField when loading term objects

* Optimistic updates

* Use per page '-1', it's okay with 'include' query