Added support for macOS and Apple Silicon through MoltenVK portability layer

[mpalomas]

Added supports for macOS and Apple Silicon through MoltenVK portability layer. Tested only on MacBook Pro M1 Pro.

• Slang is now fetched for macOS properly
• Uses Vulkan Portability extension to create instance and device
• MoltenVK does not support "component swizzle", so we need to ignore it for macOS. Apparently still works. Disclaimer: I've used Claude to fixe this error since I did not understand how to check for a portability feature availability.
• Most macOS specific code is under #ifdef because it's not guaranteed Windows and Linux SDKs offer the portability layer, so it would not even compile I guess.

There are still some validation errors, but still, it surprisingly works: