Allow free() to be used as Callable

This method is registered in a special way so ClassDB doesn't naturally
know about its existence. Here it is hardcoded if any other option fail
to check if it is about the `free()` method and, if so, say it exists
and return a Callable.

modules/gdscript/tests/scripts/runtime/features/free_is_callable.gd

@@ -0,0 +1,10 @@
+func test():
+	var node := Node.new()
+	var callable: Callable = node.free
+	callable.call()
+	print(node)
+
+	node = Node.new()
+	callable = node["free"]
+	callable.call()
+	print(node)

modules/gdscript/tests/scripts/runtime/features/free_is_callable.out

@@ -0,0 +1,3 @@
+GDTEST_OK
+<Freed Object>
+<Freed Object>