Migrate macOS workflow to iOS/tvOS workflow

[flibitijibibo]

We're a little over a year away from macOS single-assembly support ending, and I haven't seen a lot of movement to get the migration done, so here's a ticket to track that.

In short, the same environment used for iOS/tvOS should also work with macOS, we just need to update the build system ever so slightly:

• SDL2 Xcode
• FAudio Xcode-iOS/ -> Xcode/
• FNA3D Xcode-iOS/ -> Xcode/
• Theorafile Xcode-iOS/ -> Xcode/
• Build Scripts
  • Add macOS build lines
  • Update paths from Xcode-iOS/ to Xcode/ where needed
• FNA on iOS/tvOS -> FNA on macOS/iOS/tvOS

Note that while we're tracking this here, this issue is not and will not be a blocker for anything at any point, so if the migration doesn't get finished when I drop Mac support on my end, poof, Mac support will be gone just like that. We have a number of Mac developers in the community so this is definitely doable, it just involves updating a lot of boring files in the various repos.

(Somewhat related: Since Visual Studio for Mac is EOL, I'm guessing that will need to be replaced as well...?)

[flibitijibibo]

The build scripts now do macOS binaries:

https://github.com/TheSpydog/fnalibs-apple-builder

[flibitijibibo]

All of the fnalibs are ready to go, so the last item is the environment itself - this can be done after iOS/tvOS update to .NET 8 via FNA-XNA/FNA#466.