Correctly format integers when there are leading zeros with a grouping
character, such as "{:04,d}".format(0x100) -> "0,256".
The new padding patterns for commas-and-zeroes and underscores-and-zeroes
are smooshed together into the existing pad_zeroes to save space.
Only the two combinations of (decimal + commas) and (other bases +
underscores) are properly supported.
Also add a test for it.
Fixes issue #18082.
Signed-off-by: Jeff Epler <jepler@unpythonic.net>
