Desktop: Fixes #10768: Make :w trigger sync in the beta editor's Vim mode

[personalizedrefrigerator]

Summary

This pull request fixes #10768 by mapping :w to "sync" in the beta markdown editor (as it is in the legacy CodeMirror 5 editor).

Testing plan

1. Enable vim keybindings.
2. Enable the beta markdown editor.
3. Press Escape then :w.
4. Verify that the sync action is triggered.
   • For me, this resulted in the "select a sync target" dialog being shown.
5. Switch to the legacy markdown editor.
6. Repeat steps 2-3.

This has been tested successfully on Ubuntu 24.04.