Don't flag images served modern via Accept negotiation

[M-arcus]

This pull request refactors how the tool checks for modern image formats (like AVIF and WebP) to more accurately detect what image formats are actually served to users. The main logic for detecting modern image formats has been moved from the DOM-based script to a new HAR-based rule, which inspects the actual Content-Type headers in HTTP responses. This approach avoids false positives/negatives caused by relying on file extensions or DOM heuristics, and allows for correct handling of Accept-based content negotiation. The update also introduces comprehensive tests for the new logic and adds support for new image MIME types in the test webserver.

Modern image format detection overhaul:

• Replaces the DOM-based rule in lib/dom/performance/modernImageFormats.js with a new HAR-based rule in lib/har/performance/modernImageFormats.js that checks the actual Content-Type delivered, ensuring accurate identification of legacy and modern image formats regardless of URL or markup. [1] [2]

Utility enhancements:

• Adds a new util.getResourceContentType function in lib/dom/util.js to allow in-page scripts to retrieve the real Content-Type of resources using the PerformanceResourceTiming API, although this is now less central due to the HAR-based approach.

Test improvements:

• Introduces an extensive test suite for the new HAR-based modern image formats rule in test/har/performance/modernImageFormatsTest.js, covering various scenarios including content negotiation, content-type casing/parameters, partial legacy/modern mixes, missing content-types, and exclusion of tracking pixels.
• Updates the test webserver in test/help/webserver.js to serve .webp and .avif files with correct MIME types, and makes a minor refactor to directory handling logic. [1] [2]

Test cleanup:

• Removes a now-unnecessary blank line from a DOM performance test.