Lexical based editor

[ssddanbrown]

Playing with the idea of using Lexical as a new editor, to eventually replace TinyMCE. A lot of work though, not looking forward to tables :scream:. Switching direct from TinyMCE would be a hard landing, as the schema/structure imposed does is more strict than what TinyMCE would accept in our configuration, and there's bound to be many differences. Would want to release in parallel, first as opt-in, then opt-out, then remove TinyMCE.

Related to #4908