Make gen_struct_info useable for 3rd parties

[sbc100]

We've had requests over the years from @juj and also from the WebGPU folks for this. I think we are pretty close to having something that is useable.

Back in #22251 I made its so that the result of running gen_struct_info can be included in JS libraries, so the remaining part is simply ensuring (testing) that gen_struct_info can be used on 3rd party code.