Add support for 'custom' on 'macos_icon' to support a completely custom app icon

src/config/Config.zig

@@ -2735,6 +2735,8 @@ keybind: Keybinds = .{},
 ///  * `blueprint`, `chalkboard`, `microchip`, `glass`, `holographic`,
 ///    `paper`, `retro`, `xray` - Official variants of the Ghostty icon
 ///    hand-created by artists (no AI).
+///  * `custom` - Use a completely custom icon. The location must be specified
+///    using the additional `macos-custom-icon` configuration
 ///  * `custom-style` - Use the official Ghostty icon but with custom
 ///    styles applied to various layers. The custom styles must be
 ///    specified using the additional `macos-icon`-prefixed configurations.
@@ -2753,6 +2755,15 @@ keybind: Keybinds = .{},
 ///     effort.
 @"macos-icon": MacAppIcon = .official,
 
+/// The absolute path to the custom icon file.
+/// Supported formats include PNG, JPEG, and ICNS.
+///
+/// Defaults to `~/.config/ghostty/Ghostty.icns`
+///
+/// Note: This configuration is required when `macos-icon` is set to
+/// `custom`
+@"macos-custom-icon": ?[]const u8 = null,
+
 /// The material to use for the frame of the macOS app icon.
 ///
 /// Valid values:
@@ -6975,6 +6986,7 @@ pub const MacAppIcon = enum {
     paper,
     retro,
     xray,
+    custom,
     @"custom-style",
 };