WGSL: Support the requires directive

[ErichDonGubler]

Standard WGSL supports something called the requires global directive, which allows shaders to declare that certain extensions either need to be supported by the WebGPU host, or shader compilation should fail. It's used like this:

requires readonly_and_readwrite_storage_textures; // single extension
requires packed_4x8_integer_dot_product, unrestricted_pointer_parameters; // multiple extensions, comma-separated

WGSL standard link: https://www.w3.org/TR/WGSL/#requires-directive