markdown_preview: Group settings and add automatic preview opening (#63462)

## Objective

Markdown preview appearance settings are split between root theme
settings and
`markdown_preview`, and Markdown files always open as source editors
even when a user primarily
wants the rendered preview.

## Solution

- Group the Markdown preview font settings under `markdown_preview`,
migrate their legacy keys
across supported settings scopes, and update defaults, Settings UI, and
documentation.
- Add `markdown_preview.open_markdown_files_in_preview`, defaulting to
false. When enabled, clean
Markdown opens are replaced by a rendered preview while preserving dirty
editors, temporary-tab
behavior, pane activation and focus, targeted navigation, workspace
restoration, and the escape
  back to source.

## Testing

- Manually verified on macOS that enabling the setting opens Markdown
files as preview-only.
- `cargo test -p markdown_preview` (three consecutive runs)
- `cargo test -p migrator nest_markdown_preview`
- `cargo test -p settings_content`
- `cargo check -p markdown -p markdown_preview -p migrator -p settings
-p settings_content -p settings_ui -p theme_settings -p workspace`
- `./script/clippy -p gpui_apple -p markdown -p markdown_preview -p
migrator -p settings -p settings_content -p settings_ui -p
theme_settings -p workspace --features gpui_apple/runtime_shaders`
- `cargo fmt --all -- --check`
- `git diff --check`

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Recordings


https://github.com/user-attachments/assets/032ab36d-f934-4ef7-aa18-53f3955a7df8


https://github.com/user-attachments/assets/dc872265-2fe0-45ac-95eb-ff820d3a8164

Release Notes:

- Added a setting to open Markdown files directly in preview mode and
grouped Markdown preview settings together.

docs/src/visual-customization.md

@@ -78,15 +78,17 @@ If you would like to use distinct themes for light mode/dark mode that can be se
   // and any other snippet of code.
   "agent_buffer_font_size": 12,
 
-  // Controls the font size for the markdown preview.
-  // If not specified, it falls back to the editor font size.
-  "markdown_preview_font_size": null,
-  // Controls the font family for the markdown preview.
-  // If not specified, it falls back to the UI font family.
-  "markdown_preview_font_family": null,
-  // Controls the font family for code blocks in the markdown preview.
-  // If not specified, it falls back to the editor font family.
-  "markdown_preview_code_font_family": null
+  "markdown_preview": {
+    // Controls the font size for the markdown preview.
+    // If not specified, it falls back to the editor font size.
+    "font_size": null,
+    // Controls the font family for the markdown preview.
+    // If not specified, it falls back to the UI font family.
+    "font_family": null,
+    // Controls the font family for code blocks in the markdown preview.
+    // If not specified, it falls back to the editor font family.
+    "code_font_family": null
+  }
 ```
 
 ### Font ligatures