Add regenie/splitl0 module

[lyh970817]

REGENIE is a C++ program for whole-genome regression modelling of large GWAS cohorts. This PR adds regenie/splitl0, the first module in REGENIE's native chunked Step 1 level-0 workflow. The module creates the split master file and per-job SNP lists used by downstream regenie/runl0 jobs.

This PR depends on the base regenie/step1 module in #11008.

PR checklist

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests!
• If you've added a new tool - have you followed the module conventions in the contribution docs
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• Ensure that the test works with either Docker / Singularity. Conda CI tests can be quite flaky:
  • For modules:
    • nf-core modules test regenie/splitl0 --profile docker
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