LibWeb: Add ResourceLoader::load_sync()

This function creates a nested event loop and runs a load() operation
inside it, returning only once the load has either succeeded or failed.

This will be used to implement blocking loads (ew!)

Libraries/LibWeb/ResourceLoader.h

@@ -42,6 +42,7 @@ public:
     static ResourceLoader& the();
 
     void load(const URL&, Function<void(const ByteBuffer&)> success_callback, Function<void(const String&)> error_callback = nullptr);
+    void load_sync(const URL&, Function<void(const ByteBuffer&)> success_callback, Function<void(const String&)> error_callback = nullptr);
 
     Function<void()> on_load_counter_change;