Desktop: Update plugin compatibility layer to allow more legacy plugins (e.g. Markdown Prettier) to run

[personalizedrefrigerator]

Summary

This pull request adds .getDoc to the CodeMirror 5 emulation layer, which is required by the markdown-prettier plugin.

Testing plan

1. Add markdown-prettier as a development plugin.
   • Note: Locally, I tested with a version that uses a newer version of prettier and Joplin plugin template.
2. Restart Joplin.
3. Click the "format" button.
4. Verify that the note is formatted
5. Install Rich Markdown (regression testing).
6. Restart Joplin.
7. Enable focus mode.
8. Verify that non-active lines are still faded.

This has been tested successfully on Fedora 40.