RFC: Migrate CodeMirror to Monaco

[antfu]

Currently, we use CoreMirror for the in-app editor, while we support Monaco in the sides. With #1330 solving the major problem we had in Monaco, it makes it less optimal to maintain two different text editors. Now, with the Shiki Monaco integration, we could have a much better syntax highlight for the markdown content as well.

Consider the in-app editor only has a single instance and it's dev only, performance shouldn't a major concern.

This is also related to #1390