csplit doesn't support precision

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