Add dependency-free TOML parser

[rickwierenga]

Summary

• add a small dependency-free TOML parser compatible with Python 3.9
• support common values, tables, dotted keys, arrays, inline tables, comments, and date/time values
• add focused tests for parsing and error handling

Test plan

• .venv/bin/python -m pytest -q pylabrobot/lib/toml_tests.py
• .venv/bin/python -m ruff check pylabrobot/lib
• .venv/bin/python -m ruff format --check pylabrobot/lib
• .venv/bin/python -m mypy pylabrobot/lib
• Python 3.9 AST grammar check