Improve context expansion (#10957)

Release Notes:

- Improved expand excerpt indicators to allow unidirectional expansion.
Also added the `editor::ExpandExcerptsUp` and
`editor::ExpandExcerptsDown` actions, which can both take a `lines`
parameter. Also added a `expand_excerpt_lines` setting which controls
the default number of lines that the indicators and actions use.

---------

Co-authored-by: conrad <conrad@zed.dev>

crates/collab/src/tests/following_tests.rs

@@ -308,8 +308,9 @@ async fn test_basic_following(
         result
     });
     let multibuffer_editor_a = workspace_a.update(cx_a, |workspace, cx| {
-        let editor =
-            cx.new_view(|cx| Editor::for_multibuffer(multibuffer_a, Some(project_a.clone()), cx));
+        let editor = cx.new_view(|cx| {
+            Editor::for_multibuffer(multibuffer_a, Some(project_a.clone()), true, cx)
+        });
         workspace.add_item_to_active_pane(Box::new(editor.clone()), None, cx);
         editor
     });