editor: Current line highlight options (#11710)

None:

<img width="717" alt="none"
src="https://github.com/zed-industries/zed/assets/2101250/b2a741db-c64a-4275-a612-5a0d15c9cab7">

Gutter:

<img width="715" alt="gutter"
src="https://github.com/zed-industries/zed/assets/2101250/f7a68a6e-6eba-41b4-9042-5a5fe2ee21a4">

Line:

<img width="717" alt="line"
src="https://github.com/zed-industries/zed/assets/2101250/117f5b00-abd7-425b-8047-1a6fab8293a7">

All:

<img width="715" alt="all"
src="https://github.com/zed-industries/zed/assets/2101250/ebccc0da-0fa0-44e5-903c-cc49d975db76">

This PR adds the `current_line_highlight` setting that defines how to
highlight the current line in the editor:

- `none`: Don't highlight the current line.
- `gutter`: Highlight the gutter area only.
- `line`: Highlight the editor area only.
- `all` (default): Highlight the whole line.

The options have been borrowed from VSCode.

Fixes #5222
Part of #4382

Release Notes:

- Added the `current_line_highlight` setting that defines how to
highlight the current line in the editor (#5222).

assets/settings/default.json

@@ -73,6 +73,17 @@
   "drop_target_size": 0.2,
   // Whether the cursor blinks in the editor.
   "cursor_blink": true,
+  // How to highlight the current line in the editor.
+  //
+  // 1. Don't highlight the current line:
+  //    "none"
+  // 2. Highlight the gutter area:
+  //    "gutter"
+  // 3. Highlight the editor area:
+  //    "line"
+  // 4. Highlight the full line (default):
+  //    "all"
+  "current_line_highlight": "all",
   // Whether to pop the completions menu while typing in an editor without
   // explicitly requesting it.
   "show_completions_on_input": true,