editor: Inline Code Actions Indicator (#31432)

Follow up to https://github.com/zed-industries/zed/pull/30140 and
https://github.com/zed-industries/zed/pull/31236

This PR introduces an inline code action indicator that shows up at the
start of a buffer line when there's enough space. If space is tight, it
adjusts to lines above or below instead. It also adjusts when cursor is
near indicator.

The indicator won't appear if there's no space within about 8 rows in
either direction, and it also stays hidden for folded ranges. It also
won't show up in case there is not space in multi buffer excerpt. These
cases account for very little because practically all languages do have
indents.


https://github.com/user-attachments/assets/1363ee8a-3178-4665-89a7-c86c733f2885

This PR also sets the existing `toolbar.code_actions` setting to `false`
in favor of this.

Release Notes:

- Added code action indicator which shows up inline at the start of the
row. This can be disabled by setting `inline_code_actions` to `false`.

assets/settings/default.json

@@ -213,6 +213,8 @@
   // Whether to show the signature help after completion or a bracket pair inserted.
   // If `auto_signature_help` is enabled, this setting will be treated as enabled also.
   "show_signature_help_after_edits": false,
+  // Whether to show code action button at start of buffer line.
+  "inline_code_actions": true,
   // What to do when go to definition yields no results.
   //
   // 1. Do nothing: `none`
@@ -324,7 +326,7 @@
     // Whether to show agent review buttons in the editor toolbar.
     "agent_review": true,
     // Whether to show code action buttons in the editor toolbar.
-    "code_actions": true
+    "code_actions": false
   },
   // Titlebar related settings
   "title_bar": {