Don't hard code every built-in function

Source file with the built-ins: https://github.com/godotengine/godot-visual-script/blob/main/visual_script_builtin_funcs.cpp

You can see with this PR https://github.com/godotengine/godot-visual-script/pull/34 how easily this source file gets out of sync with master (~4 incompatibilities within a few weeks). Once VisualScript is rewritten with GDExtension https://github.com/godotengine/godot-visual-script/pull/26, I am assuming some magic could be done to generate these bindings.