pgsql: Fix parsing of underscores in pg_plan_advice occurrence numbers

Fix parsing of underscores in pg_plan_advice occurrence numbers

The pg_plan_advice scanner recognizes underscores as digit separators
just like the core parser, but used strtoint() to convert occurrence
numbers which does not support underscores. Consequently, advice such
as SEQ_SCAN(x#1_0) failed to parse. Fix by using pg_strtoint32_safe()
like the core scanner, and also add regression test coverage.

This bug was independently found and reported by Lukas Fittl and Chao
Li. Backpatch down to v19 where pg_plan_advice was introduced.

Author: Chao Li <lic(at)highgo(dot)com>
Co-authored-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Reported-by: Lukas Fittl <lukas(at)fittl(dot)com>
Reported-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Reviewed-by: Lukas Fittl <lukas(at)fittl(dot)com>
Discussion: https://postgr.es/m/22E2ECE0-B768-43D5-8575-61C3EBC2E4E8@gmail.com
Discussion: https://postgr.es/m/CAP53PkzKeD=t90OfeMsniYrcRe2THQbUx3g6wV17Y=ZtiwmWTQ@mail.gmail.com
Backpatch-through: 19

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/f87375bfba5cee2dcea4c19879ee990463be9636

Modified Files
--------------
contrib/pg_plan_advice/expected/syntax.out | 16 ++++++++++++++++
contrib/pg_plan_advice/pgpa_scanner.l | 17 ++++++++++++-----
contrib/pg_plan_advice/sql/syntax.sql | 6 ++++++
3 files changed, 34 insertions(+), 5 deletions(-)