refactor: use unified request interface

[ROpdebee]

Unify the fetch and GM_xmlHttpRequest interfaces into a single API, request. This will make it easier to add additional logic to requests (such as logging, mocking, etc.) and is overall just nicer to work with.

Cherry-picked from #591.

[github-actions[bot]]

:rocket: Released 2 new userscript version(s):

• mb_caa_dimensions 2023.6.9 in 6f6dd35a1f5d49d4e18288d3cf271e79c6a642b1
• mb_enhanced_cover_art_uploads 2023.6.9 in 413bcfb83ea848ccd59afe281d8ddb0da9424927