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>

assets/settings/default.json

@@ -124,6 +124,8 @@
   "wrap_guides": [],
   // Hide the values of in variables from visual display in private files
   "redact_private_values": false,
+  // The default number of lines to expand excerpts in the multibuffer by.
+  "expand_excerpt_lines": 3,
   // Globs to match against file paths to determine if a file is private.
   "private_files": [
     "**/.env*",