support SPV_KHR_ray_tracing_position_fetch in naga

[Vecvec]

Is your feature request related to a problem? Please describe.
spirv supports SPV_KHR_ray_tracing_position_fetch which can be very useful if one does not want to keep vertex & index buffers around

Describe the solution you'd like
The feature to be supported

Describe alternatives you've considered
Not supporting it, it's only on spirv - Con: uses one or two of the (valuable) buffer spaces in a shader.

Additional context
Previous suggestion for API (see #1040 (comment))

var acc_w_vertex_return: acceleration_structure<vertex_return>;
var normal_acc: acceleration_structure;
// only allowed to be initialised by acceleration_structure<vertex_return>
var rq_w_vertex_return: ray_query<vertex_return>;
// allowed to be initialised by both
var normal_rq: ray_query;

//function to get vertices from latest hit, same conditions as rayQueryGet*Intersection
fn get{Committed/Candidate}HitVertexPositions(rq: ptr<function, ray_query<vertex_return>>) -> array<vec3<f32>, 3>

I've partially implemented this already, with getCommittedHitVertexPositions implemented

[Vecvec]

I'm not entirely sure how metal works, but looking at the metal feature set tables it seems that we can access an unlimited number of buffers from a shader if they are in an argument buffer. I think this means we could emulate this on metal since we can go around the maximum requirements here. I may be wrong though as I have no metal experience.