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/intrinsics/arrayLength/via_let_complex.wgsl b/test/intrinsics/arrayLength/via_let_complex.wgsl
index 98cfdd49..bd99c11 100644
--- a/test/intrinsics/arrayLength/via_let_complex.wgsl
+++ b/test/intrinsics/arrayLength/via_let_complex.wgsl
@@ -2,9 +2,9 @@
     a : array<i32>;
 };
 
-[[group(0), binding(0)]] var<storage, read> G : S;
+@group(0) @binding(0) var<storage, read> G : S;
 
-[[stage(compute), workgroup_size(1)]]
+@stage(compute) @workgroup_size(1)
 fn main() {
   let p = &*&G;
   let p2 = &*p;