Support CodeMirror 6

[personalizedrefrigerator]

Summary

Adds support for Joplin's beta CodeMirror 6-based markdown editor (see the Joplin plugin tutorial).

To do this, it,

1. Works around an issue in the CodeMirror compatibility layer (.getDoc is undefined)
2. Updates the plugin build script to allow importing @codemirror/ packages at runtime.
3. Registers keyboard commands with a CodeMirror 6 extension when in CodeMirror 6.

Notes

• Does not commit changes to package-lock.json.
• CodeMirror 6 plugin support requires Joplin >= 2.14.6.

Testing

1. Enable the beta editor
2. Create a new 3x3 table with header using the toolbar button
3. Fill each of the cells (navigate using tab)
4. Navigate back to the beginning of the table with shift-tab
5. Minify the table using the menu item
6. Undo the minification
7. Click on the second-to-last row
8. Add a new line using ctrl-enter
9. Add a new column using the toolbar button.
10. Align the column to the left
11. Align the column to the right
12. Repeat for the legacy CodeMirror 5 editor.

This has been tested successfully in Joplin 2.14.16 on Ubuntu 23.10.

table-tools-screen-recording.mp4

[personalizedrefrigerator]

There might be import issues in older versions of Joplin -- converting to a draft for now.

Edit: This should be fixed --- CodeMirror 6 imports have been converted to dynamic requires that will only be used when running in Joplin's beta markdown editor.

[FelisDiligens]

Hi @personalizedrefrigerator,

thanks for the PR! And sorry for taking so long to respond.
The code looks fine to me and works on Joplin 2.14.19 on my side. I merged it into the develop branch and will push an update within the next days. :)