pgsql: Add pg_iswalpha() and related functions.

Add pg_iswalpha() and related functions.

Per-character pg_locale_t APIs. Useful for tsearch parsing and
potentially other places.

Significant overlap with the regc_wc_isalpha() and related functions
in regc_pg_locale.c, but this change leaves those intact for
now.

Discussion: https://postgr.es/m/0151ad01239e2cc7b3139644358cf8f7b9622ff7.camel@j-davis.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/af164f31b9f5f00561d5831a72ab91cfe091f92e

Modified Files
--------------
src/backend/regex/regc_pg_locale.c | 150 ++--------------------------------
src/backend/utils/adt/pg_locale.c | 124 ++++++++++++++++++++++++++++
src/include/utils/pg_locale.h | 12 +++
src/include/utils/pg_locale_c.h | 160 +++++++++++++++++++++++++++++++++++++
4 files changed, 302 insertions(+), 144 deletions(-)