macos: use alternate app icon for debug builds

This makes it easier to distinguish the different builds of the Ghostty
application which is especially useful when debugging.

macos/Assets.xcassets/AppIconDev.appiconset/Contents.json

@@ -0,0 +1,74 @@
+{
+  "images" : [
+    {
+      "filename" : "icon_512x512@2x@2x 1.png",
+      "idiom" : "universal",
+      "platform" : "ios",
+      "size" : "1024x1024"
+    },
+    {
+      "filename" : "icon_16x16.png",
+      "idiom" : "mac",
+      "scale" : "1x",
+      "size" : "16x16"
+    },
+    {
+      "filename" : "icon_16x16@2x@2x.png",
+      "idiom" : "mac",
+      "scale" : "2x",
+      "size" : "16x16"
+    },
+    {
+      "filename" : "icon_32x32.png",
+      "idiom" : "mac",
+      "scale" : "1x",
+      "size" : "32x32"
+    },
+    {
+      "filename" : "icon_32x32@2x@2x.png",
+      "idiom" : "mac",
+      "scale" : "2x",
+      "size" : "32x32"
+    },
+    {
+      "filename" : "icon_128x128.png",
+      "idiom" : "mac",
+      "scale" : "1x",
+      "size" : "128x128"
+    },
+    {
+      "filename" : "icon_128x128@2x@2x.png",
+      "idiom" : "mac",
+      "scale" : "2x",
+      "size" : "128x128"
+    },
+    {
+      "filename" : "icon_256x256.png",
+      "idiom" : "mac",
+      "scale" : "1x",
+      "size" : "256x256"
+    },
+    {
+      "filename" : "icon_256x256@2x@2x.png",
+      "idiom" : "mac",
+      "scale" : "2x",
+      "size" : "256x256"
+    },
+    {
+      "filename" : "icon_512x512.png",
+      "idiom" : "mac",
+      "scale" : "1x",
+      "size" : "512x512"
+    },
+    {
+      "filename" : "icon_512x512@2x@2x.png",
+      "idiom" : "mac",
+      "scale" : "2x",
+      "size" : "512x512"
+    }
+  ],
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  }
+}

macos/Ghostty.xcodeproj/project.pbxproj

@@ -868,7 +868,7 @@
 		A5B30541299BEAAB0047F10C /* Debug */ = {
 			isa = XCBuildConfiguration;
 			buildSettings = {
-				ASSETCATALOG_COMPILER_APPICON_NAME = AppIcon;
+				ASSETCATALOG_COMPILER_APPICON_NAME = AppIconDev;
 				ASSETCATALOG_COMPILER_GLOBAL_ACCENT_COLOR_NAME = AccentColor;
 				ASSETCATALOG_COMPILER_INCLUDE_ALL_APPICON_ASSETS = NO;
 				CLANG_ENABLE_MODULES = YES;