titlecase(::String) should not break words inside graphemes

[stevengj]

As mentioned in #38574 (comment), at the very least we should not consider characters in the middle of graphemes to be word separators — this PR fixes titlecase(::String) to give titlecase("bôrked") == "Bôrked" instead of "BôRked".

Also, the documentation said that the default word separator is any "non-letter" character, but actually it was !iscased. I see no reason to not actually use !isletter here.

In the future, it would be better to follow the Unicode standard for word boundaries, and ideally have a "word" iterator much like our current grapheme iterator, but this requires support from utf8proc (JuliaStrings/utf8proc#208). But the behavior in this PR is at least closer to the standard.

This is technically not backward compatible, but I think it is unlikely to be a problem — the old behavior seems more like a bug to me.

cc @rfourquet, who last touched this function in #23393.

[JeffBezanson]

I think this qualifies as a bugfix.

[stevengj]

Should be good to merge, then.

[rfourquet]

I can't remember why I chose !iscased instead of !isletter, do you have an example where this makes a difference, which could also be added as a test? Otherwise, I don't know about the isgraphemebreak function, so I can't review that part.

[stevengj]

The other letters are mainly phonetic symbols and letters from other scripts, as far as I can tell, so I don't think they will make any difference one way or the other in any situation where you want titlecase in practice.