Add macos image for building Xamarin apps

So I don’t have to run brew cask install visual-studio on every build, making each build take 10 minutes instead of 10 seconds.

Hi @azzlack.

I’ve created a feature request here https://circleci.ideas.aha.io/ideas/CCI-I-931 for this.

Please vote to show support and track updates, and please comment to clarify anything.