[webgpu] Add the `import` keyword or some equivalent

Hello W3 Consortium,
I want to work with WGSL and WebGPU but the inability to have any kind of
import or library or modulation mechanism is crippling to any real-world
development in the language. It isn't possible currently to have reasonably
sized scriptis, instead I'm forced to jump around to different commented
sections. I would like to use WGPU as a universal shading language but as
it stands the inconvenience of that (along with other tooling issues) makes
it an unsuitable replacement for tools such as Vulkan and SPIR-V. I think
that one key functionality would do a great deal to open up development as
it would allow passing around of functional libraries while also preventing
WGSL scripts from becoming too bloated.
Thank you for your time,
From Rowan Fraser