Add editor actions for moving and selecting to next / previous excerpt (#25299)

Covers part of #5129 by adding `MoveToStartOfExcerpt`,
`MoveToEndOfExcerpt`, `SelectToStartOfExcerpt`, and
`SelectToEndOfExcerpt`.

No default linux bindings yet as it's unclear what to use. Currently,
`ctrl-up` / `ctrl-down` scroll up and down by one line (see #13269).
Considering changing the meaning of those.

Mac:

* Previously `cmd-up` and `cmd-down` were `editor::MoveToBeginning` and
`editor::MoveToEnd`. In singleton editors these will behave the same as
before. In multibuffers, they will now step through excerpts instead of
jumping to the beginning / end of the multibuffer.

* `cmd-home` and `cmd-end`, often typed as `cmd-fn-left` and
`cmd-fn-right` are now `editor::MoveToBeginning` and
`editor::MoveToEnd`. This is useful in multibuffers.

Release Notes:

- Mac: `cmd-up` now moves to the previous
multibuffer excerpt start, and `cmd-down` moves to the next multibuffer
excerpt end. Within normal buffers these behave the same as before, moving
to the beginning or end.

crates/editor/src/editor.rs

@@ -9033,6 +9033,98 @@ impl Editor {
         })
     }
 
+    pub fn move_to_start_of_excerpt(
+        &mut self,
+        _: &MoveToStartOfExcerpt,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+    ) {
+        if matches!(self.mode, EditorMode::SingleLine { .. }) {
+            cx.propagate();
+            return;
+        }
+
+        self.change_selections(Some(Autoscroll::fit()), window, cx, |s| {
+            s.move_with(|map, selection| {
+                selection.collapse_to(
+                    movement::start_of_excerpt(
+                        map,
+                        selection.head(),
+                        workspace::searchable::Direction::Prev,
+                    ),
+                    SelectionGoal::None,
+                )
+            });
+        })
+    }
+
+    pub fn move_to_end_of_excerpt(
+        &mut self,
+        _: &MoveToEndOfExcerpt,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+    ) {
+        if matches!(self.mode, EditorMode::SingleLine { .. }) {
+            cx.propagate();
+            return;
+        }
+
+        self.change_selections(Some(Autoscroll::fit()), window, cx, |s| {
+            s.move_with(|map, selection| {
+                selection.collapse_to(
+                    movement::end_of_excerpt(
+                        map,
+                        selection.head(),
+                        workspace::searchable::Direction::Next,
+                    ),
+                    SelectionGoal::None,
+                )
+            });
+        })
+    }
+
+    pub fn select_to_start_of_excerpt(
+        &mut self,
+        _: &SelectToStartOfExcerpt,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+    ) {
+        if matches!(self.mode, EditorMode::SingleLine { .. }) {
+            cx.propagate();
+            return;
+        }
+
+        self.change_selections(Some(Autoscroll::fit()), window, cx, |s| {
+            s.move_heads_with(|map, head, _| {
+                (
+                    movement::start_of_excerpt(map, head, workspace::searchable::Direction::Prev),
+                    SelectionGoal::None,
+                )
+            });
+        })
+    }
+
+    pub fn select_to_end_of_excerpt(
+        &mut self,
+        _: &SelectToEndOfExcerpt,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+    ) {
+        if matches!(self.mode, EditorMode::SingleLine { .. }) {
+            cx.propagate();
+            return;
+        }
+
+        self.change_selections(Some(Autoscroll::fit()), window, cx, |s| {
+            s.move_heads_with(|map, head, _| {
+                (
+                    movement::end_of_excerpt(map, head, workspace::searchable::Direction::Next),
+                    SelectionGoal::None,
+                )
+            });
+        })
+    }
+
     pub fn move_to_beginning(
         &mut self,
         _: &MoveToBeginning,