pgsql: Fix INITCAP() word boundaries for PG_UNICODE_FAST.

Fix INITCAP() word boundaries for PG_UNICODE_FAST.

Word boundaries are based on whether a character is alphanumeric or
not. For the PG_UNICODE_FAST collation, alphanumeric includes
non-ASCII digits; whereas for the PG_C_UTF8 collation, it only
includes digits 0-9. Pass down the right information from the
pg_locale_t into initcap_wbnext to differentiate the behavior.

Reported-by: Noah Misch <noah@leadboat.com>
Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/20250417135841.33.nmisch@google.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/90260e2ec6bbfc3dfa9d9501ab75c535de52f677

Modified Files
--------------
src/backend/utils/adt/pg_locale_builtin.c  |  4 +++-
src/common/unicode/case_test.c             | 13 ++++++++++++-
src/test/regress/expected/collate.utf8.out |  8 ++++++--
src/test/regress/sql/collate.utf8.sql      |  2 ++
4 files changed, 23 insertions(+), 4 deletions(-)