Unify the HILTI/Spicy scanners & parsers

[bbannier]

The flex and bison inputs for HILTI and Spicy share a lot of logic. Currently that's all duplicated, although not consistently: there are some differences even for things where the languages are the same. We should try to find a way to unify these so that we factor out any joint pieces, and then just left HILTI/Spicy-specifics separate. The main challenge there is that (IIRC) Flex/Bison don't support any kind of "include" directives.