feat(poly check): report unused bricks in projects

Adding a feature to the poly check command: report any unused bricks in projects.

In case a project includes a brick that is no longer needed, this new feature will report on it. Currently, it is only information and won't return with a fail exit code.

Usage, examples:

# poetry
poetry poly check --strict

# uv (or any of your favorite package & dependency management tool)
uv run poly check --strict

When there's a brick not used in a specific project, the output will be:

🔎 Is <the_brick> needed in <the_project>?

Implementation details in #360