Add folder_indicator setting to show chevron and folder icon together (#62393)

# Objective

- Enable panels to show chevron and icon at the same time.
- Closes #8661

## Solution

- Replaces `folder_icon` with a new `folder_indicator` which converts
from a boolean to enum (`both`, `chevron`, `icon`) for the three display
cases

## Testing

- gpui and migration tests had been applied

## 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

## Showcase

### project panel

both | chevron | icon
--- | --- | ---
<img width="400" alt="both_project_panel"
src="https://github.com/user-attachments/assets/4c21e1a1-a7a4-4471-b7de-fe867373ddbd"
/> | <img width="400" alt="chevron_project_panel"
src="https://github.com/user-attachments/assets/a626a82e-e5b7-470d-9d93-ed5787dc83d6"
/> | <img width="400" alt="icon_project_panel"
src="https://github.com/user-attachments/assets/20f9c55e-87c4-477a-b4c9-b070d6e5a1db"
/>

### git panel

both | chevron | icon
--- | --- | ---
<img width="400" alt="both_git_panel"
src="https://github.com/user-attachments/assets/3dc166dd-9e63-4943-b803-df3f167876b6"
/> | <img width="400" alt="chevron_git_panel"
src="https://github.com/user-attachments/assets/8dfd2c10-3a68-439a-8019-4951b72e17d1"
/> | <img width="400" alt="icon_git_panel"
src="https://github.com/user-attachments/assets/d78a9938-91e7-4d39-a3c8-04647da67768"
/>

### outline panel

both | chevron | icon
--- | --- | ---
<img width="400" alt="both_outline_panel"
src="https://github.com/user-attachments/assets/1b58c679-0bea-4600-a8ac-e800b0e8d24c"
/> | <img width="400" alt="chevron_outline_panel"
src="https://github.com/user-attachments/assets/0a9358e4-ec89-40d8-8d5e-a53b784ed30a"
/> | <img width="400" alt="icon_outline_panel"
src="https://github.com/user-attachments/assets/d8bf5199-5a8c-4de0-b516-00247e6531a5"
/>



---

Release Notes:

- Added a `folder_indicator` setting that can show the disclosure
chevron and folder icon at the same time in the project, outline, and
git panels.

---------

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

docs/src/visual-customization.md

@@ -475,7 +475,7 @@ Project panel can be shown/hidden with {#action project_panel::ToggleFocus} ({#k
     "dock": "right",                // Position of the dock (left, right)
     "entry_spacing": "comfortable", // Vertical spacing (comfortable, standard)
     "file_icons": true,             // Show/hide file icons
-    "folder_icons": true,           // Show/hide folder icons
+    "folder_indicator": "icon",     // Dir glyph: icon, chevron, both
     "git_status": true,             // Indicate new/updated files
     "indent_size": 20,              // Pixels for each successive indent
     "auto_reveal_entries": true,    // Show file in panel when activating its buffer
@@ -598,7 +598,7 @@ See [Terminal settings](./reference/all-settings.md#terminal) for additional non
     "default_width": 300,         // Default width of the git panel
     "dock": "left",               // Where to dock: left, right
     "file_icons": true,           // Show/hide file_icons
-    "folder_icons": true,         // Show file_icons (true), chevrons (false) for dirs
+    "folder_indicator": "icon",   // Dir glyph: icon, chevron, both
     "git_status": true,           // Show git status
     "indent_size": 20,            // Indentation for nested items (pixels)
     "indent_guides": {