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:

squawk/crates/squawk_linter/src/lib.rs

Lines 429 to 442 in 52fdeca

	fn errors(&mut self, text: &str) -> Vec<Violation> {
	let ignore_index = IgnoreIndex::new(text, &self.ignores);
	let mut errors: Vec<Violation> = self
	.errors
	.iter()
	// TODO: we should have errors for when there was an ignore but that
	// ignore didn't actually ignore anything
	.filter(|err| !ignore_index.contains(err.text_range, err.code))
	.cloned()
	.collect::<Vec<_>>();
	// ensure we order them by where they appear in the file
	errors.sort_by_key(|x| x.text_range.start());
	errors
	}