feat: use Swift 5 and Xcode 12

[marksinkovics]

I found this great example about how to build up an app what would use plugins to extent its functionality. Thank you for making this sample, it helped a lot to understand it.

When I tried to compile, I noticed it uses Swift 3 so this PR updates the code for using Swift 5 and it also contains the recommended Xcode updates.

Thank you for reviewing and please let me know if you have any question regarding this PR.