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>

test/struct/type_constructor.wgsl

@@ -20,7 +20,7 @@ struct T {
   a : array<i32, 2>;
 };
 
-[[stage(compute), workgroup_size(1)]]
+@stage(compute) @workgroup_size(1)
 fn main() {
   let x : i32 = 42;
 

test/struct/type_constructor.wgsl.expected.wgsl

@@ -20,7 +20,7 @@ struct T {
   a : array<i32, 2>;
 }
 
-[[stage(compute), workgroup_size(1)]]
+@stage(compute) @workgroup_size(1)
 fn main() {
   let x : i32 = 42;
   let empty : S1 = S1();