Enrich with custom error types

Provide better errors, i.e.:

typedRegExp('pattern', 'gig'); // Errors: Argument of type '"gig"' is not assignable to parameter of type 'undefined'

could become:

typedRegExp('pattern', 'gig'); // Errors: Argument of type 'gig' is not assignable to parameter of type 'FlagError<"Repeated flag 'g'">'.

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