csplit doesn't support precision

[sylvestre]

With ours:

$ echo > in
$ csplit -b '%0#6.3x' in 1
csplit: incorrect conversion specification in suffix

With GNU:

$ echo > in
$ /usr/bin/csplit -b '%0#6.3x' in 1
0
1

The regexp here: https://github.com/uutils/coreutils/blob/main/src/uu/csplit/src/split_name.rs#L52 and the following code needs to be updated

Tested in GNU with tests/csplit/csplit.sh