pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/46e5441fa536b89c1123f270fdfeeb72c320b901

Modified Files
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src/backend/utils/adt/formatting.c | 107 +++++++++++++++++++++++--------------
src/common/unicode_case.c          |  76 +++++++++++++++++++++++---
src/include/common/unicode_case.h  |   5 ++
3 files changed, 140 insertions(+), 48 deletions(-)