Support Safari extensions (for macOS)

[Jaifroid]

Now that Safari supports Manifest v3 and background Service Workers, it should be pretty simple to support and publish a Safari-based extension, and makes a lot of sense sense we already support Safari when using the app as a PWA (both pwa.kiwix.org and browser-extension.kiwix.org).

The main difficulty I foresee would be testing the extension code on a mac (I don't personally have access to one, but surely another Kiwix Dev could help us out at least in terms of the initial setup), and publishing the extension. Now I don't currently know if a Safari extension can be installed from a file, or whether it requires publishing in the App Store... If it is supported from a file, we could initially start that way (gradual introduction). If only from the App Store, then the bar is a bit higher as it's already a lot of hassle dealing with three different extension stores currently. Adding a fourth might be a bit "too much" at least all in one go.

@kelson42 What would be your advice here? Note that users of macOS can already use the Firefox or Chrome extensions, but adding Safari would probably be good for the Kiwix brand's visibility.