Restore usability of codegen.ts

[tetraptych]

PR #763 fixed issue #762 by removing certain "states" from the defined states type. This created compile-time errors in zipCodesByCountyByState.ts: the output of codegen.ts no longer had the expected type, since some records in the API response no longer could be assigned the correct type.

We should allow for this file to be created from codegen directly without the need for a manual step ensuring that the problematic lines do not appear in zipCodesByCountyByState.ts.