wgsl: Replace [[decoration]] with @decoration

Deprecate the old syntax. Migrate everything to the new syntax.

Bug: tint:1382
Change-Id: Ide12b2e927b17dc93b9714c7049090864cc568d3
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/77260
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: David Neto <dneto@google.com>
diff --git a/test/ptr_ref/access/matrix.wgsl b/test/ptr_ref/access/matrix.wgsl
index 1347133..a085846 100644
--- a/test/ptr_ref/access/matrix.wgsl
+++ b/test/ptr_ref/access/matrix.wgsl
@@ -1,4 +1,4 @@
-[[stage(compute), workgroup_size(1)]]
+@stage(compute) @workgroup_size(1)
 fn main() {
   var m : mat3x3<f32> = mat3x3<f32>(vec3<f32>(1., 2., 3.), vec3<f32>(4., 5., 6.), vec3<f32>(7., 8., 9.));
   let v : ptr<function, vec3<f32>> = &m[1];