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

A key principle is to define the constraints of the language in code , e.g. [vsc_ast.py](https://github.com/COVESA/vsc-tools/blob/master:vsc/model/vsc_ast.py) but none of the code lists or limits the fundamental native datatypes as described in
[the current documentation](https://github.com/COVESA/vehicle_service_catalog/blob/master/syntax.md).

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.