Add settings to control gutter elements (#7665)

The current gutter was a bit too big for my taste, so I added some
settings to change which visual elements are shown, including being able
to hide the gutter completely.

This should help with the following issues: #4963, #4382, #7422

New settings:
```json5
"gutter": {
    "line_numbers": true, // Whether line numbers are shown
    "buttons": true // Whether the code action/folding buttons are shown
}
```

The existing `git.git_gutter` setting is also taken into account when
calculating the width of the gutter.

We could also separate the display of the code action and folding
buttons into separate settings, let me know if that is desirable.

## Screenshots:

- Everything on (`gutter.line_numbers`, `gutter.buttons`,
`git.git_gutter`):
<img width="434" alt="SCR-20240210-trfb"
src="https://github.com/zed-industries/zed/assets/17355488/bcc55311-6e1d-4c22-8c43-4f364637959b">

- Only line numbers and git gutter (`gutter.line_numbers`,
`git.git_gutter`):
<img width="406" alt="SCR-20240210-trhm"
src="https://github.com/zed-industries/zed/assets/17355488/0a0e074d-64d0-437c-851b-55560d5a6c6b">

- Only git gutter (`git.git_gutter`):
<img width="356" alt="SCR-20240210-trkb"
src="https://github.com/zed-industries/zed/assets/17355488/7ebdb38d-93a5-4e38-b008-beabf355510d">

- Only git gutter and buttons (`git.git_gutter`, `gutter.buttons`):
<img width="356" alt="SCR-20240210-txyo"
src="https://github.com/zed-industries/zed/assets/17355488/e2c92c05-cc30-43bc-9399-09ea5e376e1b">


- Nothing:
<img width="350" alt="SCR-20240210-trne"
src="https://github.com/zed-industries/zed/assets/17355488/e0cd301d-c3e0-4b31-ac69-997515928b5a">



## Release Notes:
- Added settings to control the display of gutter visual elements. `"gutter": {"line_numbers": true,    "code_actions": true,    "folds": true}` ([#8041](https://github.com/zed-industries/zed/issues/8041))  ([#7422](https://github.com/zed-industries/zed/issues/7422))
```

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/diagnostics/src/diagnostics.rs

@@ -885,7 +885,7 @@ mod tests {
     use super::*;
     use editor::{
         display_map::{BlockContext, TransformBlock},
-        DisplayPoint,
+        DisplayPoint, GutterDimensions,
     };
     use gpui::{px, TestAppContext, VisualTestContext, WindowContext};
     use language::{Diagnostic, DiagnosticEntry, DiagnosticSeverity, PointUtf16, Unclipped};
@@ -1599,8 +1599,7 @@ mod tests {
                                     .render(&mut BlockContext {
                                         context: cx,
                                         anchor_x: px(0.),
-                                        gutter_padding: px(0.),
-                                        gutter_width: px(0.),
+                                        gutter_dimensions: &GutterDimensions::default(),
                                         line_height: px(0.),
                                         em_width: px(0.),
                                         max_width: px(0.),