[DSLX:cleanup] Break parse errors / error position extraction out into own translation unit

Or maybe we can tack error position data on into the `absl::Status` metadata map even.

Right now this error functionality is kind of all splatted into `bindings.h` and friends, it should be nicely carved out similar to how `errors.h` is done, though that one drags in `ConcreteType` so that it can create type errors, so to avoid circular deps we may need to make a separate translation unit instead of adding the parse errors to that one.