[Feature Request] Allow overriding OS in RegisterUriScheme

[RuiNtD]

Hello. In my application's environment, Environment.OSVersion.Platform is returning Unix on Mac, which is causing RegisterUriScheme to throw a "No such file or directory" exception. I have a separate way of detecting Windows, Mac, and Linux in my code and it would be nice if I could pass that into RegisterUriScheme.

[Lachee]

you could make a PR to fix the registration scheme for Mac.

Environment shouldn't be returning unix for mac, that is why MacOSX exists. In saying that, I have actually not tested this library in MacOS yet.