Add extensions to OS menu bar

[nihaals]

This would be an extra option part of File, Edit, View, History, Bookmarks, Tools etc. called Extensions which would be similar to the jigsaw button by default and be sorted either with the same ordering or alphabetically (related #9399). This would make it significantly easier to invoke extensions' UI by using tools that wrap native menu bars e.g. Paletro or macOS 26' Spotlight now searching menu bar items.

This would probably give a similar outcome to #6676 and is currently part of Arc (which I found a huge upgrade from Firefox's UI where I would take a while finding the right extension in the list).