Desktop: CodeMirror 6 plugin API: Improve ability to register custom commands

[personalizedrefrigerator]

Summary

This pull request, as mentioned here, adds a new CodeMirror6-specific API for registering custom editor commands.

Specifically,

editorControl.registerCommand('nameHere', () => {
    return 'returnValue';
})

Previously, commands could only be registered through the legacy CodeMirror 5 compatibility layer.