Add keyboard shortcut for formatting and preserve cursor position

[fhammerschmidt]

Add a keyboard shortcut for formatting.

Currently, the cursor position gets lost after pressing it, which makes this feel very awkward. Try to preserve it.

This might only be solved by upgrading to CodeMirror 6 (https://codemirror.net/docs/migration/), which utilizes offsets instead of line coordinates.

[tsnobip]

the solution is likely to wrap code formatting into a transaction that'd calculate a diff between the old and the new code and let code mirror calculate the new position of the cursor accordingly.
https://discuss.codemirror.net/t/how-to-map-cursor-position-after-formatting-code/6267/6