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