Improve switching between rich and plain editing modes

[artcodespace]

Changes add:

• the ability to switch back and forth between rich and plain text modes whilst maintaining formatting (nb there is a known issue with formatting tags containing white space inside tags and how they are converted to markdown, raised here: issue in rich text editor repo). Example conversions shown below
  Starting in rich text mode, enter some formatted text, click convert
  
  Plain text contains the for
  
  matted text converted to markdown, then click convert to go back to rich text mode
  Back to the original version
  

• updates rich text composer dependency to 0.12.0 (@matrix-org/matrix-wysiwyg)

• use of conversion functions from that version to convert between rich and plain text, these are imported dynamically to avoid this code being imported when the rich text labs feature is disabled

• improves handling of enter keypresses when ctrlEnterTosend setting is true in plain text editor

• changes the message event content when using the new editor to use output from the new conversion functions

• adds tests for the changes to the plain text editor

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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Here's what your changelog entry will look like:

✨ Features

• Improve switching between rich and plain editing modes (#9776). Contributed by @alunturner.

[artcodespace]

It seems to convert strikethrough to ~~TEXT~~ and it works for sending but if the user enters that manually then no such luck

Can you confirm that you were typing that text into the plain text editor (ie when the formatting buttons are not showing above the composer box)? When I type that text (in between the backticks) into the plain editor and send it then it is displays as struck through when sent.

I have spoken to the team to confirm that this is the desired behaviour, but it does raise a good point about what should happen if the user types valid markdown into the rich text editor.

Edit: realised I also have the Composer => enable markdown setting set to true as well, when this setting is false, it causes the displayed message (when sent from the plain text composer) not to be interpreted as markdown

[andybalaam]

Approved, thanks!