[CCranney]

Adds visionOS as a build platform, so GDExtensions can target Apple Vision Pro. Godot has exported to visionOS since 4.5, and 4.8 adds immersive rendering through the visionOS XR module, but godot-cpp can't build for it yet: scons platform=visionos is rejected.

NOTE: It may be helpful to create a shared helper for iOS and visionOS, since they are quite similar. However, I thought that might be best done in a later PR to isolate iOS changes.

Testing

• Built with scons platform=visionos target=template_debug and target=template_release on macOS v27.2.
• Built and ran godot-cpp's test project on Apple Vision Pro (Godot 4.8.beta1, windowed). The extension and godot-cpp were packaged as .xcframeworks, as on iOS. 137 of 138 checks pass. The only failure is FileAccess.file_exists(library_path) (main.gd, line 301). I believe that would be expected of iOS as well.
• I was able to get Terrain3D running on a visionOS window app usingn these edits. My visionOS immersion app test had problems, specifically exposing an error in the Godot engine (see visionOS: Immersive rendering freezes when a material reads the depth or screen texture godot#125166), so am addressing that there.

[oPinon]

This change looks good to me.

It is in line with what we did for the GDExtensions GodotRealityKit and GodotPhase; but having it directly in Godot-cpp sounds better, indeed.