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/assistant2/src/tools/annotate_code.rs

@@ -253,7 +253,7 @@ impl ToolView for AnnotationResultView {
                 MultiBuffer::new(0, language::Capability::ReadWrite).with_title(String::new())
             });
             let editor = cx.new_view(|cx| {
-                Editor::for_multibuffer(multibuffer.clone(), Some(self.project.clone()), cx)
+                Editor::for_multibuffer(multibuffer.clone(), Some(self.project.clone()), true, cx)
             });
 
             self.editor = Some(editor.clone());