[iOS/macOS] Fix for Xcode 12.5

[kyle-seongwoo-jun]

Problem

Crash on native (iOS, macOS) when callbackURLScheme is an entire URL like foo://bar.

References: auth0/react-native-auth0#360, auth0/react-native-auth0#369

[LinusU]

Would you mind explaining a bit more why this is needed? The argument is called callbackURLScheme so I think it's implied that you should pass a scheme and not an entire url? 🤔

Could we instead detect if an entire url is passed and raise an appropriate error? 🤔

[kyle-seongwoo-jun]

@LinusU I just update the code to validate the scheme in authenticate().

Please check it out. Thank you.

[LinusU]

Hey @kyle-seongwoo-jun,

1. It seems like you copied some code from the dart core project, have you looked into the licensing/copyright implications of doing so?
2. Could we keep the schema-validator in a separate file?

[LinusU]

Thanks for taking the time to submit this PR! 🙏

Since the last comments wasn't addressed I implemented this using a regex instead: 4f89925

Happy hacking!