Add SPIR-V extensions

[MaikKlein]

What is the best way to tell rlsl which extensions should be enabled? We still need to stay compatible with rustc/cargo.

Also extensions should be enabled per shader module, and not the whole project. That means we also need a way to make it easy to share the configuration between different shader modules.

Most extensions should be trivial to support. All features should be enabled by default and a simple pass can check which features are currently used in a shader module, and outputs an error if the extension hasn't been enabled, but it used.

[MaikKlein]

#66