Meta: Add an option for building against AppKit

The build assumed QT or AppKit are the only build UI frameworks. This
extends the default assumption away from that to start experimenting
with building on other platforms.

Meta/gn/secondary/Ladybird/BUILD.gn

@@ -1,4 +1,5 @@
 import("//Ladybird/compile_qt_resource_file.gni")
+import("//Ladybird/enable_appkit.gni")
 import("//Ladybird/enable_qt.gni")
 import("//Ladybird/link_qt.gni")
 import("//Ladybird/moc_qt_objects.gni")
@@ -110,7 +111,7 @@ executable("ladybird_executable") {
       ":compile_resource_file",
       ":generate_moc",
     ]
-  } else {
+  } else if (enable_appkit) {
     sources += [
       "AppKit/Application/Application.mm",
       "AppKit/Application/ApplicationBridge.cpp",

Meta/gn/secondary/Ladybird/enable_appkit.gni

@@ -0,0 +1,4 @@
+declare_args() {
+  # Build the Ladybird application using the AppKit chrome.
+  enable_appkit = current_os == "mac"
+}