Add conjunction parameter to natural_list

[SAY-5]

Fixes #214

Changes proposed in this pull request:

• Add a keyword-only conjunction parameter to natural_list, defaulting to "and" so existing behaviour is unchanged.
• This implements the design hugovk suggested in the issue, allowing any string such as "or" or "and also".
• Add tests covering the new parameter alongside the default.

[codspeed]

Merging this PR will not alter performance

✅ 15 untouched benchmarks

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_{Comparing SAY-5:feat-natural-list-conjunction (6eb4231) with main (392aef7)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.69%. Comparing base (392aef7) to head (6eb4231).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #316   +/-   ##
=======================================
  Coverage   99.69%   99.69%           
=======================================
  Files          12       12           
  Lines         996      999    +3     
=======================================
+ Hits          993      996    +3     
  Misses          3        3           

Flag	Coverage Δ
macos-latest	97.59% <100.00%> (+<0.01%)	⬆️
ubuntu-latest	97.59% <100.00%> (+<0.01%)	⬆️
windows-latest	93.29% <100.00%> (+0.02%)	⬆️

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[SAY-5]

Any interest in this? It adds a conjunction argument to natural_list so callers can get "or" instead of the hardcoded "and".