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 #14128 (comment 210481647) ux-research#870 (closed)