Fixes to excerpt movement actions and bindings + add multibuffer and singleton_buffer key contexts (#26264)

Closes #26002 

Release Notes:

- Added `multibuffer` key context.
- `cmd-down` and `cmd-shift-down` on Mac now moves to the end of the
last line of a singleton buffer instead of the beginning. In
multibuffers, these now move to the start of the next excerpt.
- Fixed `vim::PreviousSectionEnd` (bound to `[ ]`) to move to the
beginning of the line, matching the behavior of `vim::NextSectionEnd`.
- Added `editor::MoveToStartOfNextExcerpt` and
`editor::MoveToEndOfPreviousExcerpt`.

crates/editor/src/editor.rs

@@ -1573,13 +1573,16 @@ impl Editor {
             }
         }
 
-        if let Some(extension) = self
-            .buffer
-            .read(cx)
-            .as_singleton()
-            .and_then(|buffer| buffer.read(cx).file()?.path().extension()?.to_str())
-        {
-            key_context.set("extension", extension.to_string());
+        if let Some(singleton_buffer) = self.buffer.read(cx).as_singleton() {
+            if let Some(extension) = singleton_buffer
+                .read(cx)
+                .file()
+                .and_then(|file| file.path().extension()?.to_str())
+            {
+                key_context.set("extension", extension.to_string());
+            }
+        } else {
+            key_context.add("multibuffer");
         }
 
         if has_active_edit_prediction {
@@ -9845,6 +9848,31 @@ impl Editor {
         })
     }
 
+    pub fn move_to_start_of_next_excerpt(
+        &mut self,
+        _: &MoveToStartOfNextExcerpt,
+        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::Next,
+                    ),
+                    SelectionGoal::None,
+                )
+            });
+        })
+    }
+
     pub fn move_to_end_of_excerpt(
         &mut self,
         _: &MoveToEndOfExcerpt,
@@ -9870,6 +9898,31 @@ impl Editor {
         })
     }
 
+    pub fn move_to_end_of_previous_excerpt(
+        &mut self,
+        _: &MoveToEndOfPreviousExcerpt,
+        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::Prev,
+                    ),
+                    SelectionGoal::None,
+                )
+            });
+        })
+    }
+
     pub fn select_to_start_of_excerpt(
         &mut self,
         _: &SelectToStartOfExcerpt,
@@ -9891,6 +9944,27 @@ impl Editor {
         })
     }
 
+    pub fn select_to_start_of_next_excerpt(
+        &mut self,
+        _: &SelectToStartOfNextExcerpt,
+        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::Next),
+                    SelectionGoal::None,
+                )
+            });
+        })
+    }
+
     pub fn select_to_end_of_excerpt(
         &mut self,
         _: &SelectToEndOfExcerpt,
@@ -9912,6 +9986,27 @@ impl Editor {
         })
     }
 
+    pub fn select_to_end_of_previous_excerpt(
+        &mut self,
+        _: &SelectToEndOfPreviousExcerpt,
+        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::Prev),
+                    SelectionGoal::None,
+                )
+            });
+        })
+    }
+
     pub fn move_to_beginning(
         &mut self,
         _: &MoveToBeginning,