Add PostgreSQL 19 compatibility

[sibowu-aws]

PG19 made two changes that broke pg_bigm's existing PG18 fallback for
t_isspace() (originally added in commit which copied the PG17
implementation after PG18 removed t_isspace via core commit d3aad4ac57c - postgres/postgres@d3aad4a):

• the database_ctype_is_c global flag is gone; the equivalent
  information now lives on pg_locale_t->ctype_is_c, retrievable
  via pg_database_locale().
• char2wchar() is no longer exported from utils/pg_locale.h
  (it was made static in src/backend/utils/adt/pg_locale_libc.c).

Add a PG_VERSION_NUM >= 190000 branch to t_isspace() that uses the new
public API: pg_database_locale() for the locale, pg_mb2wchar_with_len()
to decode the multibyte character, and pg_iswspace() for the classify
call. This mirrors the pattern PG19 itself uses in
src/backend/tsearch/ts_locale.c for t_isalnum / t_isalpha. The PG18
code path is preserved unchanged.

Also add alternate expected outputs (expected/pg_bigm_2.out,
expected/pg_bigm_ja_2.out) so the regression suite passes on PG19,
where:

• escape_string_warning was removed (SET errors out).
• standard_conforming_strings can no longer be turned off
  (SET errors out, and the dependent \-escape tests behave
  differently under standard strings).
• the "function does not exist" error switched its trailing
  line from HINT to DETAIL with a new message.

The original expected files are kept, so PG <= 18 continues to pass.

[devrimgunduz]

RPM tracker: pgdg-packaging/pgdg-rpms#213

[MasaoFujii]

@sibowu-aws
Thanks for working on PostgreSQL 19 support!

I prepared an alternative patch here:
master...MasaoFujii:pg_bigm:support-pg19

It uses the same basic approach for the PG19 t_isspace() path described in
this PR, but differs in a few points:

• it fixes the LockBuffer() link failure on PG19 by calling the exported
  LockBufferInternal() function instead
• it also fixes pg_gin_pending_stats() by calling TupleDescFinalize()
  before BlessTupleDesc() on PG19
• it avoids adding expected/pg_bigm_2.out and
  expected/pg_bigm_ja_2.out by making the regression SQL output less
  version-dependent
• it avoids the version-dependent function does not exist error text by
  checking pg_proc
• it updates the docs and CI matrix for PostgreSQL 19

Regards,