Show highlighted symbol in the scrollbar (#7029)

Release Notes:

- Added highlighted symbols to the scrollbar; partially mentioned in:
  - https://github.com/zed-industries/zed/issues/5308
  - https://github.com/zed-industries/zed/issues/4866

assets/settings/default.json

@@ -111,7 +111,9 @@
     // Whether to show git diff indicators in the scrollbar.
     "git_diff": true,
     // Whether to show selections in the scrollbar.
-    "selections": true
+    "selections": true,
+    // Whether to show symbols selections in the scrollbar.
+    "symbols_selections": true
   },
   "relative_line_numbers": false,
   // When to populate a new search's query based on the text under the cursor.