[wgsl-reader] Add workgroup_size parsing

This CL adds parsing of the `workgroup_size` function decoration.

Change-Id: Ia90efc2c014ac0e1614429280cc903d30cf8171d
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/28663
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: Sarah Mashayekhi <sarahmashay@google.com>

test/function.wgsl

@@ -15,3 +15,9 @@
 fn main() -> f32 {
     return ((2. * 3.) - 4.) / 5.;
 }
+
+[[workgroup_size(2)]]
+fn ep() -> void {
+  return;
+}
+entry_point compute = ep;