Add .xcodeproj and Makefile

[sbooth]

Right now the project consists of a single xcframework target with multiplatform (macOS, iOS, tvOS) support.

One issue with this PR is:

/***/SFBAudioEngine/Sources/SFBAudioEngine/SFBAudioPlayer.swift:68:38 'retroactive' attribute does not apply; 'PlaybackState' is declared in this module; this is an error in the Swift 6 language mode

Removing @retroactive generates a warning with the SPM build.

[sbooth]

The issue with Swift 6 and @retroactive is no longer a problem with Xcode 26.

[cherishloveyou]

can make a single SFBAudioEngine.XCFramework for work? too mang related package Dependencies (about 19+), can simple make all in one?

[sbooth]

You should be able to use the xcodeproj branch for that until/if this PR is merged. Just check it out and type make at the root level of the repo and you'll end up with an XCFramework in the build/ directory.

Just to expand on this a little, iOS doesn't support umbrella frameworks so there is no easy way to embed the dependencies into the generated XCFramework. On macOS it is possible but I haven't looked at this branch in a while and can't remember exactly what this does.