* Install the Mac Catalyst versions of the mono libraries and BCL.
* The BCL is the same as the one for Xamarin.iOS, which means it has to be post-processed a bit to work with a Xamarin.MacCatalyst.dll
* Build our runtime for Mac Catalyst.
* Build a Xamarin.MacCatalyst.dll with the Mac Catalyst API (it compiles, but I haven't looked at the API surface at all). This PR assumes we're going to have a new TargetFrameworkIdentifier for Mac Catalyst, but a final decision has not been made (see https://github.com/dotnet/runtime/issues/44882), so this may change.
* Build a Xamarin.iOS.dll that contains type forwarders to Mac Catalyst for all the types that exist in both Mac Catalyst and Xamarin.iOS.
* Add support to xharness for running introspection on Mac Catalyst (there are a lot of failures because the API surface is wrong)
* Add support to our msbuild tasks and mtouch for building Mac Catalyst apps. This basically comes down to adding a new case in numerous places to either do things the iOS way or the macOS way, depending on each case.
* Add a __MACCATALYST__ define (which is in addition to the __IOS__ define).
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