Widget richtext

[demshy]

Summary
This is a work-in-progress PR that adds richtext-widget as a potential replacement for the markdown in one of the future releases. Discussion where I put some of my thoughts about this here: #6905
Digging deeper, there might be another caveat that we haven't considered before - nesting block types (inside lists for example) might make the implementation much more difficult so I did not go down that road yet.

A very rough checklist:

• create new widget and install Plate
• inline marks (bold, italics, and code)
• headings
• number and bullet lists
• quote block
• links
• editor components / shortcodes
• toggle markdown view
• images
• (new/bonus) tables
• (new/bonus) drag&drop blocks

I have copied serializers (and their tests) from the existing markdown widget and am updating them as I go. I copy the renderers one by one as I get to the plugin in question.
Once the basic implementation is done, I plan on going through the e2e tests and update what is needed on the code or tests side, depends on the specific situation.

Note that this is very much in progress but I'm happy of any feedback, especially from someone who dealt with Slate/Plate before.

Test plan

Checklist

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