editor: Add scroll_beyond_last_line setting (#11155)

Add `scroll_beyond_last_line` setting with 3 options: 

- `one_page`: The default (current) behaviour of scrolling one more page
beyond the last line.
<img width="568" alt="SCR-20240429-sxry"
src="https://github.com/zed-industries/zed/assets/126383/1effbee9-759f-4858-9022-83bbb208ef82">

- `off`: No scrolling beyond the last line. 
<img width="568" alt="SCR-20240429-syhv"
src="https://github.com/zed-industries/zed/assets/126383/5391b1d7-918d-43f3-8a6f-7642ef32d174">

- `vertical_scroll_margin`: Scroll beyond the last line by the same
number of lines as `vertical_scroll_margin`. Matches the behaviour of
keyboard scrolling.
<img width="568" alt="SCR-20240429-sypc"
src="https://github.com/zed-industries/zed/assets/126383/bb9cc928-e515-4503-88f7-e434c45d742f">

Release Notes:

- Added `scroll_beyond_last_line` setting
([#4962](https://github.com/zed-industries/zed/issues/4962)).

assets/settings/default.json

@@ -255,6 +255,8 @@
     /// 2. "indent_aware"
     "background_coloring": "disabled"
   },
+  // Whether the editor will scroll beyond the last line.
+  "scroll_beyond_last_line": "one_page",
   // The number of lines to keep above/below the cursor when scrolling.
   "vertical_scroll_margin": 3,
   // Scroll sensitivity multiplier. This multiplier is applied