Rework hunk controls (#24814)

- Remove prev hunk arrow
- Replace next hunk arrow with "Skip" labelled button
- New "Stage"/"Unstage" labelled button

cc @iamnbutler 

Release Notes:

- N/A

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Co-authored-by: Nate <nate@zed.dev>
Co-authored-by: Nate Butler <iamnbutler@gmail.com>