Add non-ASCII case conversion to String#upcase/downcase/swapcase/capitalize

[deepj]

It's same issue as in Ruby. See https://bugs.ruby-lang.org/issues/10085 and https://github.com/ruby/ruby/blob/trunk/NEWS

Basically, the current implementation of case conversion strings is currently limited to ASCII characters.

Now (Crystal 0.18.2, note lower-case ü):

 "Türkiye".upcase # =>  TüRKIYE

Would be:

"Türkiye".upcase # =>  TÜRKIYE

[asterite]

Yes, I think we'll eventually do it, but it's very hard.

Duplicate of #1648

[deepj]

@asterite Ah, thanks for notice, second duplicate here :)

[asterite]

@deepj No worries about duplicates, sometimes they are hard to find because different words are used. Duplicates mean there's interest on those issues so they help :-)