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 #34 how easily this source file gets out of sync with master (~4 incompatibilities within a few weeks). Once VisualScript is rewritten with GDExtension #26, I am assuming some magic could be done to generate these bindings.