Flag unused ignores with a warning

[reteps]

If you add an ignore that isn't needed, I would expect that to raise a warning.

[sbdchd]

Yeah makes sense!

I think we'd need to update the following and basically mark which ignores have been used as we go through the errors:

https://github.com/sbdchd/squawk/blob/52fdeca5d5466957cc21be33bf399fc7bd763949/crates/squawk_linter/src/lib.rs#L429-L442