Refactor reusable block edit component using hooks (and fix interacti…

…ons with multiple instances of the same reusable block) (#21427)

* Refactor reusable block edit component using hooks.

* Only use local state while editing.

* Use __experimentalGetParsedReusableBlock.

* Attempt to fix temporary reusable block behavior.

* Fix mometary emptiness on start/stop editing.