Added VisionOS as a supported target to the Xcode project

Also added SDL_PLATFORM_VISIONOS to the platform definitions and generally switched from TARGET_OS_* macros to SDL_PLATFORM_* macros.

include/SDL3/SDL_platform_defines.h

@@ -86,13 +86,16 @@
 #ifndef TARGET_OS_SIMULATOR
 #define TARGET_OS_SIMULATOR 0
 #endif
-#ifndef TARGET_OS_XR
-#define TARGET_OS_XR 0
+#ifndef TARGET_OS_VISION
+#define TARGET_OS_VISION 0
 #endif
 
 #if TARGET_OS_TV
 #define SDL_PLATFORM_TVOS   1
 #endif
+#if TARGET_OS_VISION
+#define SDL_PLATFORM_VISIONOS 1
+#endif
 #if TARGET_OS_IPHONE
 #define SDL_PLATFORM_IOS    1
 #else