Integrate custom text editor

[lorefnon]

Thanks for creating this extension. I have been refraining from using vscode based notes management utilities because I really liked the integrated rendering in editors like typora, but now having a single application for code, notes, diagrams through excalidraw/draw.io integrations is really nice.

I saw the note about custom editor integration in README, and thought it would be a useful addition to make the integration more frictionless. Let me know if you have any feedback - I don't have any prior experience with vscode extensions.