Modernize _parse_ext to handle multi-extension filenames

[scott-huberty]

Related to #1528

_parse_ext splits a filename into its stem and extension. It already contained a special case for .nii.gz files. here I refactor it to generally handle multi-extension filenames (EDIT: because I need _parse_ext to handle .tsv.gz files).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.98%. Comparing base (8e52e3e) to head (8332acb).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1536      +/-   ##
==========================================
- Coverage   97.00%   96.98%   -0.02%     
==========================================
  Files          43       43              
  Lines       10692    10697       +5     
==========================================
+ Hits        10372    10375       +3     
- Misses        320      322       +2     

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[scott-huberty]

Here's why I think this test was weird and why I think it warrants a change.

copyfile_eeglab will throw an error if dest contains a different extension than src.

On main, fname == "test_raw.set" and new_name == "/CONVERTED_test_raw.set.set")

So on L231, we do copyfile_eeglab(raw_fname, new_name)

and inside copyfile_eeglab, we do _parse_ext("/CONVERTED_test_raw.set.set") which returns ('/CONVERTED_test_raw.set', '.set') ..

I think this just happened to work because now the files end up with the same extension even though _parse_ext didn't really do its job, and even though the caller passed a dest with a different extension than src.