[url_launcher] Open url in same window when browser is in standalone mode.

[tugorez]

Description

Users using this plugin on iOS PWAs are experiencing blank screens when a url is opened on another app and then they try to go back. It seems to be an well known issue on iOS [1] and [2] as reference. In the meantime we can overcome this limitation by modifying the target to "_top" according to [2] (solution manually verified). This PR does exactly that, verifies if the user is running a PWA in iOS and if so it will open the url with "_top" as target.

[1] https://stackoverflow.com/questions/55517297/mailto-doesnt-work-when-website-launched-from-ios-home-screen/56153685#56153685
[2] https://stackoverflow.com/questions/56220862/ios-pwa-open-maps-returns-to-blank-screen

Related Issues

flutter/flutter#51461

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[ditman]

This looking great, I've added only a few comments!

[ditman]

I tested this with my phone and an emulator and it seems to fix the issue! I'll merge this as soon as the tree goes green again.

Thanks for the contribution @tugorez!