Improve line commenting on diffs with keyboard navigation
## Proposal
Allow for consistent keyboard navigation when commenting on lines on a diff.
1. For single line editing, currently works for initial comment but when you "cancel" the comment, it no longer works.
- This could be an existing bug
2. For multi line editing, we will need be able to select multiple lines.
- `Shift` + `up`/`down`: Select multiple lines
- `Enter`: open comment with selected range
There were past discussions on making this feature of line commenting to have keyboard navigation https://gitlab.com/gitlab-org/gitlab/-/issues/14128#note_210481647 https://gitlab.com/gitlab-org/ux-research/-/issues/870
issue