Fix String#underscore with multi-character downcasing

[HertzDevil]

This is similar to #13539, although it really only affects strings that contain İ and do not use Unicode::CaseOptions::Turkic.

Since the compiler uses String#underscore to generate question methods for enums as well, there is a non-zero but negligible chance this could break someone's code:

enum Foo
  Barİ
end

Foo::Barİ.bari?  # no longer compiles
Foo::Barİ.bar_i̇? # will compile