Add line range editing mode to Rapid Diffs comments

Adds multi-line comment line range editing to Rapid Diffs, part of #602600 (closed).

An edit button on the comment form opens an editing mode where the commented line range can be adjusted with draggable handles or the keyboard.

Reproduction

1. Open a merge request's Changes tab (Rapid Diffs) and start a comment on a diff line.
   • Ensure that the rapid_diffs_on_mr_show feature flag is enabled.
   • Click the Try Rapid Diffs button in the header.
2. Use the pencil on the comment form to edit the range, or drag to select multiple lines.
3. Adjust with the drag handles or Up/Down arrow keys; confirm with Enter or drop, cancel with Escape or the cancel control.