Enrich with custom error types

Provide better errors, i.e.:
```ts
typedRegExp('pattern', 'gig'); // Errors: Argument of type '"gig"' is not assignable to parameter of type 'undefined'
```
could become:
```ts
typedRegExp('pattern', 'gig'); // Errors: Argument of type 'gig' is not assignable to parameter of type 'FlagError<"Repeated flag 'g'">'.
```



