feat: improved detection whether a string requires all matches

String sets like `($s*)` always removed the `STRING_FLAGS_SINGLE_MATCH` flag, but only some operators (e.g. `1 of ($s*) at 0`) actually mandated this. In practice, the most common use case of string sets is `n of ($s*)`, which does not necessitate removing `STRING_FLAGS_SINGLE_MATCH`. Add logic to the operators to only remove this flag where it is necessary.