Use dedicated classes instead of arrays with special keys

[oliverklee]

This will make the code more explicit and help static code analysis.

Particularly, we should get rid of compact, list and family.

[SignpostMarv]

it appears that CssConcatenator::getOrCreateMediaRuleToAppendTo() may require a dedicated class in order to satisfy some MixedPropertyFetch issues in psalm: https://github.com/MyIntervals/emogrifier/blob/6f9d875f0054ab3d8cd6a972a7d517ced19a2040/src/Utilities/CssConcatenator.php#L103