LibJS: Add Object::put_native_function() for convenience

This makes it a little bit nicer to add native function properties
to JavaScript objects.

Thanks to Sergey for suggesting it! :^)

Libraries/LibJS/Object.cpp

@@ -25,6 +25,8 @@
  */
 
 #include <AK/String.h>
+#include <LibJS/Heap.h>
+#include <LibJS/NativeFunction.h>
 #include <LibJS/Object.h>
 #include <LibJS/Value.h>
 
@@ -48,6 +50,11 @@ void Object::put(String property_name, Value value)
     m_properties.set(property_name, move(value));
 }
 
+void Object::put_native_function(String property_name, AK::Function<Value(Interpreter&, Vector<Value>)> native_function)
+{
+    put(property_name, heap().allocate<NativeFunction>(move(native_function)));
+}
+
 void Object::visit_children(Cell::Visitor& visitor)
 {
     Cell::visit_children(visitor);