editor: Add minimap (#26893)

## Overview

This PR adds the minimap feature to the Zed editor, closely following
the [design from Visual Studio
Code](https://code.visualstudio.com/docs/getstarted/userinterface#_minimap).
When configured, a second instance of the editor will appear to the left
of the scrollbar. This instance is not interactive and it has a slimmed
down set of annotations, but it is otherwise just a zoomed-out version
of the main editor instance. A thumb shows the line boundaries of the
main viewport, as well as the progress through the document. Clicking on
a section of code in the minimap will jump the editor to that code.
Dragging the thumb will act like the scrollbar, moving sequentially
through the document.

![screenshot of Zed with three editors open and the minimap enabled,
showing the
slider](https://github.com/user-attachments/assets/4178d23a-a5ea-4e38-b871-06dd2a8f9560)

## New settings

This adds a `minimap` section to the editor settings with the following
keys:

### `show`

When to show the minimap in the editor.
This setting can take three values:
1. Show the minimap if the editor's scrollbar is visible: `"auto"`
2. Always show the minimap: `"always"`
3. Never show the minimap: `"never"` (default)

### `thumb`

When to show the minimap thumb.
This setting can take two values:
1. Show the minimap thumb if the mouse is over the minimap: `"hover"`
2. Always show the minimap thumb: `"always"` (default)

### `width`

The width of the minimap in pixels.

Default: `100`

### `font_size`

The font size of the minimap in pixels.

Default: `2`

## Providing feedback

In order to keep the PR focused on development updates, please use the
discussion thread for feature suggestions and usability feedback: #26894


## Features left to add

- [x] fix scrolling performance
- [x] user settings for enable/disable, width, text size, etc.
- [x] show overview of visible lines in minimap
- [x] clicking on minimap should navigate to the corresponding section
of code
- ~[ ] more prominent highlighting in the minimap editor~
- ~[ ] override scrollbar auto setting to always when minimap is set to
always show~

Release Notes:

- Added minimap for high-level overview and quick navigation of editor
contents.

---------

Co-authored-by: MrSubidubi <dev@bahn.sh>
Co-authored-by: Kirill Bulatov <kirill@zed.dev>

assets/settings/default.json

@@ -356,6 +356,49 @@
       "vertical": true
     }
   },
+  // Minimap related settings
+  "minimap": {
+    // When to show the minimap in the editor.
+    // This setting can take three values:
+    // 1. Show the minimap if the editor's scrollbar is visible:
+    //    "auto"
+    // 2. Always show the minimap:
+    //    "always"
+    // 3. Never show the minimap:
+    //    "never" (default)
+    "show": "never",
+    // When to show the minimap thumb.
+    // This setting can take two values:
+    // 1. Show the minimap thumb if the mouse is over the minimap:
+    //    "hover"
+    // 2. Always show the minimap thumb:
+    //    "always" (default)
+    "thumb": "always",
+    // How the minimap thumb border should look.
+    // This setting can take five values:
+    // 1. Display a border on all sides of the thumb:
+    //    "thumb_border": "full"
+    // 2. Display a border on all sides except the left side of the thumb:
+    //    "thumb_border": "left_open" (default)
+    // 3. Display a border on all sides except the right side of the thumb:
+    //    "thumb_border": "right_open"
+    // 4. Display a border only on the left side of the thumb:
+    //    "thumb_border": "left_only"
+    // 5. Display the thumb without any border:
+    //    "thumb_border": "none"
+    "thumb_border": "left_open",
+    // How to highlight the current line in the minimap.
+    // This setting can take the following values:
+    //
+    // 1. `null` to inherit the editor `current_line_highlight` setting (default)
+    // 2. "line" or "all" to highlight the current line in the minimap.
+    // 3. "gutter" or "none" to not highlight the current line in the minimap.
+    "current_line_highlight": null,
+    // The width of the minimap in pixels.
+    "width": 100,
+    // The font size of the minimap in pixels.
+    "font_size": 2
+  },
   // Enable middle-click paste on Linux.
   "middle_click_paste": true,
   // What to do when multibuffer is double clicked in some of its excerpts