Warn on unused `ignore`s from `pyproject.toml`

I’m on mobile so I can’t confirm but I think M001 doesn’t warn about unused excludes in the config. I think it should since it's essentially the same reasoning as unused per-line ignores. Thoughts?

Start by tracing how M001 handles per-line ignores and how exclusions are read from pyproject.toml. Determine how the linter can identify configured excludes that were never used; done means unused excludes produce warnings with behavior consistent with unused per-line ignores.

Written by the indexing model from the issue text.