Feat: add typing to the package

[elliot-100]

Needs PR #276 merged first.

This PR:

• Adds mypy as a development dependency with a fairly lax initial baseline configuration
• Also extends ruff ruleset to require type hints (except for args and **kwargs which I will come back to)
• Adds type hints and type: ignore[] comments to get to that baseline
• Enforces that baseline in CI (also cleans up comments on other checks)
• Documents how to run ruff and mypy checks in development
• Marks the package as typed so consumers can type check it in their applications