Rewrite with declarative APIs

[fregante]

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/contentScripts/register

https://developer.chrome.com/extensions/declarativeContent#type-RequestContentScript

https://developer.chrome.com/extensions/permissions#event-onAdded

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Edit: Done in refined-github/refined-github#2181

I'll extract it from there at some point