pgsql: Back-patch pg_encoding_verifymbstr()/pg_encoding_verifymbchar()

Back-patch pg_encoding_verifymbstr()/pg_encoding_verifymbchar() to v13.

A security fix will need those functions, so back-patch the v14+ functions to
v13.

When commit b80e10638e36b9d2f0b39170c613837af2ca2aac introduced the v14+
implementation of pg_encoding_verifymbstr(), it added a callback to each
pg_wchar_table entry. For simplicity and ABI stability, this instead
implements the function in terms of the existing per-character callback.

Author: Noah Misch <noah(at)leadboat(dot)com>
Author: Andres Freund <andres(at)anarazel(dot)de>
Security: CVE-2025-1094

Branch
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/00f1a1f665f078f5abadbf8baddc5c187fba80f8

Modified Files
--------------
src/common/wchar.c | 66 +++++++++++++++++++++++++++++++++++++++++++++++
src/include/mb/pg_wchar.h | 2 ++
2 files changed, 68 insertions(+)