Unify default value parsing with core string parser

[ssalbdivad]

Currently we have a regex to parse UnitLiterals and handle default values in string like:

type({
  foo: "string = 'foo'",
});

This is fine, but we could giver better error messages if we used our core parser to handle this, then performed some additional check at the top-level to make sure we didn't parse a top-level default value.

Either that or we come up with an acceptable definition for a default value on a shallow type, but it feels messy.

Either way the rules would be identical but we could give better errors when e.g. we try to use a default value before the end of the string.