MacPDF: Move some files around

1. main.m is now main.mm
2. MainMenu.xib is now no longer in a Base.lproj subfolder
3. Remove SerenityPDF.entitlements since it won't be used in our CMake
   build

Meta/Lagom/Contrib/MacPDF/main.mm

@@ -0,0 +1,16 @@
+//
+//  main.m
+//  SerenityPDF
+//
+//  Created by Nico Weber on 7/22/23.
+//
+
+#import <Cocoa/Cocoa.h>
+
+int main(int argc, char const* argv[])
+{
+    @autoreleasepool {
+        // Setup code that might create autoreleased objects goes here.
+    }
+    return NSApplicationMain(argc, argv);
+}