[wgsl-reader] Parse stage decoration

This CL adds parsing of the stage decoration to the WGSL parser. The
decoration is not hooked up yet so it's effectively ignored.

Change-Id: I8d86c55cee189f993c10b6da31a9c388ba452021
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/28664
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: David Neto <dneto@google.com>

test/function.wgsl

@@ -16,6 +16,7 @@ fn main() -> f32 {
     return ((2. * 3.) - 4.) / 5.;
 }
 
+[[stage(compute)]]
 [[workgroup_size(2)]]
 fn ep() -> void {
   return;