Add String.titlecase/2

[ukrbublik]

String.titlecase/2 is similar to String.capitalize/2 but it doesn't downcase the rest of the string.

Closes #9999

[josevalim]

💚 💙 💜 💛 ❤️

[ukrbublik]

@josevalim

Unicode says "HH" is not titlecased, which means we need
to downcase the rest of the string, which is the behaviour
we already have in capitalize.

According to this, titlecase differs from uppercase.
There are special chars, eg.
dž (lowercased) -- Dž (titlecased) -- DŽ (uppercased)
So we need to have titlecase function.
Maybe it should work with grapheme instead of binary?

[ukrbublik]

Or it should work with binaries but in more smart way - for each word in string.
However there are caveats, like:
"In Search of Sunrise" - we shouldn't titlecase "of" to "Of"
"McDonalds" - not "Mcdonalds"
"O'Brien" - not "O'brien"

[harrisi]

Sorry for bringing this old PR up, but, did this never make it into a release?

[NickNeck]

@harrisi the merged PR was reverted, see above.

[kipcole9]

I have a lib that implements the Unicode case folding algorithm should you need it (it also does locale specific case folding and other stuff): https://hexdocs.pm/unicode_string/readme.html#case-mapping