Extern definitions for WebGPU

[kainino0x]

Closure doesn't yet have extern definitions for WebGPU. Over in Emscripten, we are finally switching to externs instead of doing ["this"] everywhere (except everywhere we forgot and so it's broken). https://github.com/emscripten-core/emscripten/pull/21144

There you can see some barebones externs I generated from the WebGPU spec's WebIDL. I think it's enough for minification to work correctly(?) but it doesn't provide much else - no function arguments, in particular.

Would there be interest in bringing this into Closure as-is, or would it need more complete definitions?