Add SwiftUI visionOS example.

[halmueller]

This PR adds a standalone SwiftUI sample (ExampleVisionOS) that creates a volumetric window on visionOS.

Of note:

• Vision Pro (native) run destination has been removed from the existing Example app. The RealityKit view does not display anything in native mode. Vision Pro (Designed for iPad) works fine, and remains as a destination.
• EuclidMesh.swift in ExampleVisionOS is virtually identical to the one in Example. UIKit dependency has been replaced by CoreGraphics, and the solid UIKit color has been replaced by a solid Euclid-defined color.
• This sample only displays a volumetric window. It's cut down from the Xcode 15.3 template to be as simple and short as possible.
• I removed the bones of the Apple template relating to the unimplemented immersive experience, because that added a top-level Packages folder to the Euclid project file. I thought that was potentially confusing and distracting. The best way to provide an immersive example would be as a freestanding .xcodeproj that imports Euclid and doesn't mess with the Euclid library project structure.
• If you want to see the distracting modification that I rejected, add a visionOS target to the project file, specifying an immersive app. Note the Packages folder that appears at top level.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.46%. Comparing base (d3afbeb) to head (afaec04).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #115 +/- ## ========================================== + Coverage 77.36% 77.46% +0.09% ========================================== Files 32 32 Lines 8218 8218 ========================================== + Hits 6358 6366 +8 + Misses 1860 1852 -8 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.