Allow underscores in integer and numeric constants.
This allows underscores to be used in integer and numeric literals,
and their corresponding type input functions, for visual grouping.
For example:
1_500_000_000
3.14159_26535_89793
0xffff_ffff
0b_1001_0001
A single underscore is allowed between any 2 digits, or immediately
after the base prefix indicator of non-decimal integers, per SQL:202x
draft.
Peter Eisentraut and Dean Rasheed
Discussion: https://postgr.es/m/
84aae844-dc55-a4be-86d9-
4f0fa405cc97%40enterprisedb.com