New WYSIWYG Editor Development API

[ssddanbrown]

This issue is for sketching out some initial development API requirements for the new WYSIWYG editor.
Related to #5631.

We should document this API within the project /dev/docs.
We should probably abstract the underlying lexical stuff as much as possible, at least for common requirements, otherwise exposing so many required internals could get quite messy.

Abilities

• Custom button
• Main toolbar manipulation (add custom buttons)
• Editor actions:
  • Insert HTML at selection
    • Option for block-level content? To ensure it's added at top-level.