Encode fundamental a.k.a. "native" datatypes in the code

[gunnarx]

A key principle is to define the constraints of the language in code , e.g. vsc_ast.py but none of the code lists or limits the fundamental native datatypes as described in
the current documentation.

I propose some list of datatypes is added to vsc_ast.py so that everything is in one place, and that some form of validation is added using that list.