Add conjunction option to natural_list

[zihjiang]

Summary

Add a configurable conjunction to natural_list().

Changes

• Add a conjunction parameter defaulting to "and".
• Support custom conjunctions such as "or" and "and also" for two-item and longer lists.
• Preserve existing behavior for empty and single-item iterables.
• Document the new parameter and usage example.

Tests

• uv run pytest -q — 729 passed, 74 skipped
• uvx ruff check src/humanize/lists.py tests/test_lists.py
• uvx ruff format --check src/humanize/lists.py tests/test_lists.py
• uvx prek run --files src/humanize/lists.py tests/test_lists.py
• git diff --check

Fixes #214

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.57%. Comparing base (3201e70) to head (30092ce).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #398   +/-   ##
=======================================
  Coverage   99.56%   99.57%           
=======================================
  Files          12       12           
  Lines         927      931    +4     
=======================================
+ Hits          923      927    +4     
  Misses          4        4           

Flag	Coverage Δ
macos-latest	97.63% <100.00%> (+0.01%)	⬆️
ubuntu-latest	97.63% <100.00%> (+0.01%)	⬆️
windows-latest	95.48% <100.00%> (+0.01%)	⬆️

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