GDScript: Allow utility functions to be used as Callable

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

@@ -0,0 +1,10 @@
+func test():
+	print(print)
+	print(len)
+
+	prints.callv([1, 2, 3])
+	print(mini.call(1, 2))
+	print(len.bind("abc").call())
+
+	const ABSF = absf
+	print(ABSF.call(-1.2))

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

@@ -0,0 +1,7 @@
+GDTEST_OK
+@GlobalScope::print (Callable)
+@GDScript::len (Callable)
+1 2 3
+1
+3
+1.2