Unify AST processing between functional expressions, tuple expression, strings, standardize runtime semantic validation

[ssalbdivad]

After the refactor is complete, search repo for this issue's link and address any TODOs referencing it.

Also, add:

= (Default value) (should also work in string) Merge (maybe use "+", should not only be tuple expression)

[ssalbdivad]

Should also allow flat tuple expressions like:

const t = type(["string", "|", "boolean", "|", "number"])

[ssalbdivad]

Also consider reintegrating static satisifiability validation, e.g. for type("string&number"). This could be useful at least to do shallowly to help with cases like morphs.

It was a pain maintaining it across strings and tuple expressions, but may be more viable once this is implemented.