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/rust_analyzer_ext.rs

@@ -109,7 +109,9 @@ pub fn expand_macro_recursively(
                 MultiBuffer::singleton(buffer, cx).with_title(macro_expansion.name)
             });
             workspace.add_item_to_active_pane(
-                Box::new(cx.new_view(|cx| Editor::for_multibuffer(multibuffer, Some(project), cx))),
+                Box::new(
+                    cx.new_view(|cx| Editor::for_multibuffer(multibuffer, Some(project), true, cx)),
+                ),
                 None,
                 cx,
             );