editor: Add horizontal scrollbar (#19495)

![editor_scrollbars](https://github.com/user-attachments/assets/76c26776-8fe4-47f8-9c79-9add7d7d2151)

Closes #4427 

Release Notes:

- Added a horizontal scrollbar to the editor panel
- Added `axis` option to `scrollbar` in the Zed configuration, which can
forcefully disable either the horizontal or vertical scrollbar
- Added `horizontal_scroll_margin` equivalent to
`vertical_scroll_margin` in the Zed configuration

Rough Edges:

This feature seems mostly stable from my testing. I've been using a
development build for about a week with no issues. Any feedback would be
appreciated. There are a few things to note as well:

1. Scrolling to the lower right occasionally causes scrollbar clipping
on my end, but it isn't consistent and it isn't major. Some more testing
would definitely be a good idea. [FIXED]
2. Documentation may need to be modified
3. I added an `AxisPair` type to the `editor` crate to manage values
that have a horizontal and vertical variant. I'm not sure if that's the
optimal way to do it, but I didn't see a good alternative. The `Point`
type would technically work, but it may cause confusion.

---------

Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>

assets/settings/default.json

@@ -254,7 +254,14 @@
     // Whether to show selected symbol occurrences in the scrollbar.
     "selected_symbol": true,
     // Whether to show diagnostic indicators in the scrollbar.
-    "diagnostics": true
+    "diagnostics": true,
+    /// Forcefully enable or disable the scrollbar for each axis
+    "axes": {
+      /// When false, forcefully disables the horizontal scrollbar. Otherwise, obey other settings.
+      "horizontal": true,
+      /// When false, forcefully disables the vertical scrollbar. Otherwise, obey other settings.
+      "vertical": true
+    }
   },
   // Enable middle-click paste on Linux.
   "middle_click_paste": true,
@@ -304,6 +311,8 @@
   "vertical_scroll_margin": 3,
   // Whether to scroll when clicking near the edge of the visible text area.
   "autoscroll_on_clicks": false,
+  // The number of characters to keep on either side when scrolling with the mouse
+  "horizontal_scroll_margin": 5,
   // Scroll sensitivity multiplier. This multiplier is applied
   // to both the horizontal and vertical delta values while scrolling.
   "scroll_sensitivity": 1.0,