Add setting for enabling server-side decorations (#39250)

Previously, this was controllable via the undocumented
ZED_WINDOW_DECORATIONS environment variable (added in #13866). Using an
environment variable for this is inconvenient because it requires users
to set that environment variable somehow before starting Zed, such as in
the .desktop file or persistently in their shell. Controlling this via a
Zed setting is more convenient.

This does not modify the design of the titlebar in any way. It only
moves the existing option from an environment variable to a Zed setting.

Fixes #14165

Client-side decorations (default):
<img width="3840" height="2160" alt="image"
src="https://github.com/user-attachments/assets/525feb92-2f60-47d3-b0ca-47c98770fa8c"
/>


Server-side decorations in KDE Plasma:
<img width="3840" height="2160" alt="image"
src="https://github.com/user-attachments/assets/7379c7c8-e5e3-47ba-a3ea-4191fec9434d"
/>

Release Notes:

- Changed option for Wayland server-side decorations from an environment
variable to settings.json field

---------

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

assets/settings/default.json

@@ -175,6 +175,16 @@
   //
   // Default: true
   "zoomed_padding": true,
+  // What draws Zed's window decorations (titlebar):
+  // 1. Client application (Zed) draws its own window decorations
+  //    "client"
+  // 2. Display server draws the window decorations. Not supported by GNOME Wayland.
+  //    "server"
+  //
+  // This requires restarting Zed for changes to take effect.
+  //
+  // Default: "client"
+  "window_decorations": "client",
   // Whether to use the system provided dialogs for Open and Save As.
   // When set to false, Zed will use the built-in keyboard-first pickers.
   "use_system_path_prompts": true,