fix(lint/noDuplicateAtImportRules): correctly handle both single- and double quotes

[DerTimonius]

Summary

While checking different rules, I noticed that I forgot to check for a specific case when implementing the noDuplicateAtImportRules rule.

@import "a.css";
@import 'a.css';

This should show an error, but didn't.

This PR fixes this small bug and also adds a test for this scenario

Test Plan

Added a new test to validate the change.

[codspeed-hq]

CodSpeed Performance Report

Merging #2707 will not alter performance

_{Comparing DerTimonius:fix/no-duplicate-at-imports (18cf153) with main (1b63093)}

Summary

✅ 86 untouched benchmarks