Ruby doesn't allow forward references

[Aloso]

Describe the bug

Fuzzing revealed that Ruby/oniguruma rejects forward references. The information here is either outdated or just wrong.

Pomsky should disallow forward references in the Ruby flavor.

To Reproduce

regex-test "(\k<x>(?<x>))+" -fruby

Pomsky expression:

:(::x? :x("a")){3}

Expected behavior

Pomsky should show an error message when targeting Ruby.