Re-forbid underscore in positional parameters

Re-forbid underscore in positional parameters

Underscores were added to numeric literals in faff8f8e47.  This change
also affected the positional parameters (e.g., $1) rule, which uses
the same production for its digits.  But this did not actually work,
because the digits for parameters are processed using atol(), which
does not handle underscores and ignores whatever it cannot parse.

The underscores notation is probably not useful for positional
parameters, so for simplicity revert that rule to its old form that
only accepts digits 0-9.

Author: Erik Wienhold <ewie@ewie.name>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://www.postgresql.org/message-id/flat/5d216d1c-91f6-4cbe-95e2-b4cbd930520c%40ewie.name