Desktop: Allow creating plugins that process pasted text in the beta editor

[personalizedrefrigerator]

Summary

This pull request adds the input.paste user event to insertText calls while pasting into the beta markdown editor.

Notes:

• This is what CodeMirror does internally.
  • On desktop, we implement our own paste logic to handle pasting resources and do other processing, so don't use the built-in CodeMirror paste logic.
• This should allow implementing features similar to this one ("fast links") as plugins.

Testing plan

Regression testing (for insertText):

1. Verify that paste still works:
   • Take a screenshot and paste it into the beta editor.
   • Copy text from the beta editor, paste it into the beta editor.
2. Verify that insertText still works
   • Attach a file using the toolbar button (which uses insertText to include the link).

Notes:

• insertText has automated tests in CodeMirrorControl.test.ts