Add support for visionOS

[calda]

This PR updates the structure of the lottie-spm package to support visionOS, and update CI to build for visionOS.

Lottie supports Xcode 14.1 and later. visionOS is supported by Xcode 15.2 and later. We ship a precompiled XCFramework bundle, so to support visionOS we must provide and use an XCFramework built by Xcode 15.2.

Xcode 14 can't use an XCFramework built by Xcode 15.2. Instead, we have to provide two XCFramework bundles:

1. An XCFramework built by Xcode 15.2, which supports visionOS and can be used by Xcode 15.0+
2. An XCFramework built by Xcode 14.1, which doesn't support visionOS but can be used by Xcode 14.

Following this policy, Lottie will only support Xcode 15.0+ starting in April 2024. At that point we can remove the additional XCFramework for Xcode 14, and instead just use the XCFramework built by Xcode 15.2.

A sidenote: I personally expected Xcode 15.0 to be unable to use an XCFramework built by Xcode 15.2, but all the testing I've done so far (including in GitHub actions CI) says that this works correctly. If this turns out to be an issue in practice we'll have to tweak the version gating.

[calda]

Deleted the separate iOS (UIKit) and macOS (AppKit) example apps in favor of a single cross-platform SwiftUI example app.

When we get ready to merge this we'll want to add CI jobs that test all supported platforms on both Xcode 14 and Xcode 15.

[karanshah]

Hello! You can add a multiple swift Package files to support different swift versions. See this article. Also here is another example from an open source project. Hopefully this helps bringing in support for Swift 5.9

[calda]

The visionOS build will fail until we update this to a build of Lottie 4.4.0 (not released yet), but I tested this in CI with a custom XCFramework bundle built by Xcode 15.2 and confirmed that the current setup works: