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/editor/src/hunk_diff.rs

@@ -572,7 +572,7 @@ fn editor_with_deleted_text(
             );
         });
 
-        let mut editor = Editor::for_multibuffer(multi_buffer, None, cx);
+        let mut editor = Editor::for_multibuffer(multi_buffer, None, true, cx);
         editor.soft_wrap_mode_override = Some(language::language_settings::SoftWrap::None);
         editor.show_wrap_guides = Some(false);
         editor.show_gutter = false;