refactor[next]: Use type specification for itir.Literal

[tehrengruber]

Small PR in preparation of the new ITIR type system. Currently the type of a itir.Literal is stored as a string which blocks introducing a type: ts.TypeSpecification attribute in all itir.Nodes. In order to keep the PR for the new type inference easy to review this has been factored out.

class Literal(Expr):
    value: str
    type: str

    @datamodels.validator("type")
    def _type_validator(self: datamodels.DataModelTP, attribute: datamodels.Attribute, value):
        if value not in TYPEBUILTINS:
            raise ValueError(f"'{value}' is not a valid builtin type.")

is changed to

class Literal(Expr):
    value: str
    type: ts.ScalarType

[egparedes]

Minor suggestions to but overall, it looks good. I won't need another review so I'm approving the PR now but please don't forget to fix them.