Add unicode_strtitle() for Unicode Default Case Conversion.

Add unicode_strtitle() for Unicode Default Case Conversion.

This brings the titlecasing implementation for the builtin provider
out of formatting.c and into unicode_case.c, along with
unicode_strlower() and unicode_strupper(). Accepts an arbitrary word
boundary callback.

Simple for now, but can be extended to support the Unicode Default
Case Conversion algorithm with full case mapping.

Discussion: https://postgr.es/m/3bc653b5d562ae9e2838b11cb696816c328a489a.camel@j-davis.com
Reviewed-by: Peter Eisentraut