project_panel: Add sort mode (#40160)

Closes #4533 (partly at least)

Release Notes:

- Added `project_panel.sort_mode` option to control explorer file sort
(directories first, mixed, files first)

 ## Summary

Adds three sorting modes for the project panel to give users more
control over how files and directories are displayed:

- **`directories_first`** (default): Current behaviour - directories
grouped before files
- **`mixed`**: Files and directories sorted together alphabetically
- **`files_first`**: filed grouped before directories

 ## Motivation

Users coming from different editors and file managers have different
expectations for file sorting. Some prefer directories grouped at the
top (traditional), while others prefer the macOS Finder-style mixed
sorting where "Apple1/", "apple2.tsx" and "Apple3/" appear
alphabetically mixed together.


 ### Screenshots

New sort options in settings:
<img width="515" height="160" alt="image"
src="https://github.com/user-attachments/assets/8f4e6668-6989-4881-a9bd-ed1f4f0beb40"
/>


Directories first | Mixed | Files first
-------------|-----|-----
<img width="328" height="888" alt="image"
src="https://github.com/user-attachments/assets/308e5c7a-6e6a-46ba-813d-6e268222925c"
/> | <img width="327" height="891" alt="image"
src="https://github.com/user-attachments/assets/8274d8ca-b60f-456e-be36-e35a3259483c"
/> | <img width="328" height="890" alt="image"
src="https://github.com/user-attachments/assets/3c3b1332-cf08-4eaf-9bed-527c00b41529"
/>


### Agent usage

Copilot-cli/claude-code/codex-cli helped out a lot. I'm not from a rust
background, but really wanted this solved, and it gave me a chance to
play with some of the coding agents I'm not permitted to use for work
stuff

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>

assets/settings/default.json

@@ -742,6 +742,16 @@
       //    "never"
       "show": "always"
     },
+    // Sort order for entries in the project panel.
+    // This setting can take three values:
+    //
+    // 1. Show directories first, then files:
+    //    "directories_first"
+    // 2. Mix directories and files together:
+    //    "mixed"
+    // 3. Show files first, then directories:
+    //    "files_first"
+    "sort_mode": "directories_first",
     // Whether to enable drag-and-drop operations in the project panel.
     "drag_and_drop": true,
     // Whether to hide the root entry when only one folder is open in the window.