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/editor_test_context.rs

@@ -22,6 +22,7 @@ use std::{
         Arc,
     },
 };
+
 use ui::Context;
 use util::{
     assert_set_eq,
@@ -149,6 +150,10 @@ impl EditorTestContext {
         self.multibuffer(|buffer, cx| buffer.snapshot(cx).text())
     }
 
+    pub fn display_text(&mut self) -> String {
+        self.update_editor(|editor, cx| editor.display_text(cx))
+    }
+
     pub fn buffer<F, T>(&mut self, read: F) -> T
     where
         F: FnOnce(&Buffer, &AppContext) -> T,