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/test.rs

@@ -39,6 +39,8 @@ pub fn marked_display_snapshot(
             font,
             font_size,
             None,
+            true,
+            1,
             1,
             1,
             FoldPlaceholder::test(),
@@ -74,7 +76,7 @@ pub fn assert_text_with_selections(
 #[allow(dead_code)]
 #[cfg(any(test, feature = "test-support"))]
 pub(crate) fn build_editor(buffer: Model<MultiBuffer>, cx: &mut ViewContext<Editor>) -> Editor {
-    Editor::new(EditorMode::Full, buffer, None, cx)
+    Editor::new(EditorMode::Full, buffer, None, true, cx)
 }
 
 pub(crate) fn build_editor_with_project(
@@ -82,7 +84,7 @@ pub(crate) fn build_editor_with_project(
     buffer: Model<MultiBuffer>,
     cx: &mut ViewContext<Editor>,
 ) -> Editor {
-    Editor::new(EditorMode::Full, buffer, Some(project), cx)
+    Editor::new(EditorMode::Full, buffer, Some(project), true, cx)
 }
 
 #[cfg(any(test, feature = "test-support"))]