So VS Code is going to be more than a text editor... if I were going to make a custom editor, living in VS Code would be attractive. Looks like one must write it in TS/JS. "We continued to work on the custom editor proposal this iteration. (https://github.com/microsoft/vscode/issues/77131) As a reminder, custom editors are webview-based views that can be used in place of VS Code's normal text editor for specific file types. As of VS Code 1.41, Custom editors can now integrate with VS Code's Undo/Redo and Save functionality, which enables many interesting use cases including using them as visual and WYSIWYG editors. You can find the current API proposal in 

vscode.proposed.d.ts

(https://github.com/microsoft/vscode/blob/master/src/vs/vscode.proposed.d.ts) and we have also put together some very simple extension samples that demonstrate using custom editors for text and for binary files. (https://github.com/mjbvz/vscode-experimental-webview-editor-extension) If you are interested in custom editors, please share your feedback on the current proposal and let us know if you would be able to implement it."

This is very interesting. If anyone here starts tinkering with this, please share your repos.