This implements some of #10457.

Release notes:

- Added `g c c` and `g c` to Vim keybindings to toggle comments in
normal and visual mode respectively.
- Added `g ]` and `g [` to Vim keybindings to go to next and previous
diagnostic error.
- Changed `[ x` and `] x` (which select larger/smaller syntax node) in
Vim mode to also work in visual mode.