Add basic Xamarin.Mac support

[chucker]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

This uses NSLog on macOS the same way it was already used on iOS.

Does this PR introduce a breaking change?

Possibly!

I've rewritten the iOS project to use the new Sdk project style, and I've removed Xamarin-specific dependencies as they weren't being used and impeded the build process.

Therefore, you should test that logging from iOS actually still works. It should, though.

Please check if the PR fulfills these requirements

• [x] The commit follows our guidelines: https://github.com/serilog/serilog#contribute
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

[RLittlesII]

@chucker Thank you for the contribution. Have you had the opportunity to test these changes? I will do my best to pull them down this week and give them a look.

[chucker]

@RLittlesII I have tested it on macOS. However, I have not been able to test if it breaks iOS.

[RLittlesII]

@chucker I am bringing this into a feature branch into the main repository so I can iterate on some clean up.

Thanks for your contribution!