Add ability to separately set background color for highlighted brackets (#17566)

Closes https://github.com/zed-industries/zed/issues/16380

Currently brackets are highlighted with
`editor.document_highlight.read_background`. This commit adds a separate
`editor.document_highlight.bracket_background` theme setting so bracket
highlights can be made more prominent without doing the same to other
highlights, making the display too busy.

(My own theme)


https://github.com/user-attachments/assets/29a8c05e-2f1a-4c16-9be8-a4b4cb143548

I set defaults for light and dark theme that I hope are sensible and not
too obnoxious, but noticeable so people can change it if they don't like
it.

Release Notes:

- Added `editor.document_highlight.bracket_background` field to the
theme to set background color of highlighted brackets.
- This will fall back to `editor.document_highlight.read_background`, if
not set.

<img width="355" alt="Screenshot 2024-09-08 at 8 46 57 AM"
src="https://github.com/user-attachments/assets/3270bb4d-19f5-4b34-8003-982377b2ceb6">
<img width="444" alt="Screenshot 2024-09-08 at 9 03 27 AM"
src="https://github.com/user-attachments/assets/3b12d84d-913c-4bde-9132-9b10f4a8d49b">

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/editor/src/highlight_matching_bracket.rs

@@ -32,7 +32,7 @@ pub fn refresh_matching_bracket_highlights(editor: &mut Editor, cx: &mut ViewCon
                 opening_range.to_anchors(&snapshot.buffer_snapshot),
                 closing_range.to_anchors(&snapshot.buffer_snapshot),
             ],
-            |theme| theme.editor_document_highlight_read_background,
+            |theme| theme.editor_document_highlight_bracket_background,
             cx,
         )
     }