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/auto_update/src/auto_update.rs

@@ -237,8 +237,9 @@ fn view_release_notes_locally(workspace: &mut Workspace, cx: &mut ViewContext<Wo
                             let buffer = cx.new_model(|cx| MultiBuffer::singleton(buffer, cx));
 
                             let tab_description = SharedString::from(body.title.to_string());
-                            let editor = cx
-                                .new_view(|cx| Editor::for_multibuffer(buffer, Some(project), cx));
+                            let editor = cx.new_view(|cx| {
+                                Editor::for_multibuffer(buffer, Some(project), true, cx)
+                            });
                             let workspace_handle = workspace.weak_handle();
                             let view: View<MarkdownPreviewView> = MarkdownPreviewView::new(
                                 MarkdownPreviewMode::Default,