As originally promised in the TypeSafe Errors PR, this PR dramatically simplifies all the ParseError machinery now that we are on TypeScript (woooo!!!). From a usability perspective, it is now much easier to define error templates, no longer requiring the use of the weird _() flow-workaround-function. So, before you'd have to do something like this:

const TSErrors = ParseErrorEnum`typescript` (
  (_: typeof toParseErrorCredentials) => ({
    AbstractMethodHasImplementation: _<{
      methodName: string;
    }>(
      ({ methodName }) =>
        `Method '${methodName}' cannot have an implementation because it is marked abstract.`,
    ),
    AccesorCannotDeclareThisParameter: _(
      "'get' and 'set' accessors cannot declare 'this' parameters.",
    ),
    // ...
  }),
);

And now you instead do this:

const TSErrors = ParseErrorEnum`typescript` (
  AbstractMethodHasImplementation: ({ methodName }:  { methodName: string }) =>
    `Method '${methodName}' cannot have an implementation because it is marked abstract.`,
  AccesorCannotDeclareThisParameter: "'get' and 'set' accessors cannot declare 'this' parameters.",
  // ...
});

In other words, we've kept the simplicity of the old object reason-code to message mapping, but with real types. This was of course the original goal, but many flow bugs prevented us from doing this as cleanly, so we needed these weird "carrier" functions (like toParseErrorCredentials) to shuttle the types around. TypeScript is much better at tracking the types everywhere and so we no longer need them and can so we can just use normal objects.

Aside from that, there are just a few random fixes to get rid of ts-expect-errors, etc., and updating comments to reflect the new code.