Add an apply button to hunks in proposed changes editor (#18592)

Release Notes:

- N/A

---------

Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Nathan <nathan@zed.dev>

crates/editor/src/editor.rs

@@ -6205,6 +6205,20 @@ impl Editor {
         }
     }
 
+    fn apply_selected_diff_hunks(&mut self, _: &ApplyDiffHunk, cx: &mut ViewContext<Self>) {
+        let snapshot = self.buffer.read(cx).snapshot(cx);
+        let hunks = hunks_for_selections(&snapshot, &self.selections.disjoint_anchors());
+        self.transact(cx, |editor, cx| {
+            for hunk in hunks {
+                if let Some(buffer) = editor.buffer.read(cx).buffer(hunk.buffer_id) {
+                    buffer.update(cx, |buffer, cx| {
+                        buffer.merge_into_base(Some(hunk.buffer_range.to_offset(buffer)), cx);
+                    });
+                }
+            }
+        });
+    }
+
     pub fn open_active_item_in_terminal(&mut self, _: &OpenInTerminal, cx: &mut ViewContext<Self>) {
         if let Some(working_directory) = self.active_excerpt(cx).and_then(|(_, buffer, _)| {
             let project_path = buffer.read(cx).project_path(cx)?;