Add support for Firefox and Safari in options import/export

[fregante]

Follows:

• Import/export refined github settings #3981
• Add options export/import #6448

Includes:

• https://github.com/fregante/webext-options-sync/pull/65/files/06fafa8a6f002908e2ef43683a4d8181baf612da..7cf1c25080528a177192d8563b68c5a68afb5f46

Working at first try 😍 I also tested importing a file from Chrome

Screen.Recording.1.mov

[fregante]

Of course Safari is in its own league. The file won't be saved but it's just shown and it's up to the user to save it. Even FileSaver.js fails here because it's uses Blob internally.

The closest I got to saving was by using application/octet-stream but it was saved as Unknown.txt, which is worse, because it requires a manual rename.

• Chrome: Show "Save dialog"
• Firefox: Automatically save in Downloads folder
• Safari: Shows JSON