plpgsql: fix parsing of integer range with underscores

plpgsql fails to parse 1_000..1_000 as 1000..1000 in FOR loops:

DO $$
DECLARE
i int;
BEGIN
FOR i IN 1_000..1_000 LOOP
END LOOP;
END $$;

ERROR: syntax error at or near "1_000."
LINE 5: FOR i IN 1_000..1_000 LOOP

The scan.l defines rule "numericfail" to handle this ambiguity without
requiring extra whitespace or parenthesis around the integer literals.
But the rule only accepts digits 0-9. Again, an oversight in
faff8f8e47. Fixed in the attached patch.

--
Erik