feat: 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