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-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks

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_{Comparing SAY-5:feat-natural-list-conjunction (0cedecf) with main (bde649f)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.55%. Comparing base (bde649f) to head (0cedecf).

Additional details and impacted files

@@ Coverage Diff @@## main #316 +/- ##
=======================================
Coverage 99.55% 99.55% =======================================
Files 12 12 Lines 898 901 +3 =======================================
+ Hits 894 897 +3 
Misses 4 4 

Flag	Coverage Δ
macos-latest	97.55% <100.00%> (+<0.01%)	⬆️
ubuntu-latest	97.55% <100.00%> (+<0.01%)	⬆️
windows-latest	95.67% <100.00%> (+0.01%)	⬆️

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