Fix PL/pgSQL's handling of integer ranges containing underscores.

Fix PL/pgSQL's handling of integer ranges containing underscores.

Commit faff8f8e47 allowed integer literals to contain underscores, but
failed to update the lexer's "numericfail" rule. As a result, a
decimal integer literal containing underscores would fail to parse, if
used in an integer range with no whitespace after the first number,
such as "1_001..1_003" in a PL/pgSQL FOR loop.

Fix and backpatch to v16, where support for underscores in integer
literals was added.

Report and patch by Erik Wienhold.

Discussion: https://postgr.es/m/808ce947-46ec-4628-85fa-3dd600b2c154%40ewie.name