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

---------

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

crates/editor/src/editor.rs

@@ -6975,10 +6975,10 @@ impl Editor {
         cx: &mut Context<Self>,
     ) {
         let selections = self.selections.all(cx).into_iter().map(|s| s.range());
-        self.revert_hunks_in_ranges(selections, window, cx);
+        self.discard_hunks_in_ranges(selections, window, cx);
     }
 
-    fn revert_hunks_in_ranges(
+    fn discard_hunks_in_ranges(
         &mut self,
         ranges: impl Iterator<Item = Range<Point>>,
         window: &mut Window,