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/expressions/binary/mul/vec3-scalar/f32.wgsl

@@ -1,4 +1,4 @@
-[[stage(compute), workgroup_size(1)]]
+@stage(compute) @workgroup_size(1)
 fn f() {
     let a = vec3<f32>(1., 2., 3.);
     let b = 4.;

test/expressions/binary/mul/vec3-scalar/f32.wgsl.expected.wgsl

@@ -1,4 +1,4 @@
-[[stage(compute), workgroup_size(1)]]
+@stage(compute) @workgroup_size(1)
 fn f() {
   let a = vec3<f32>(1.0, 2.0, 3.0);
   let b = 4.0;

test/expressions/binary/mul/vec3-scalar/i32.wgsl

@@ -1,4 +1,4 @@
-[[stage(compute), workgroup_size(1)]]
+@stage(compute) @workgroup_size(1)
 fn f() {
     let a = vec3<i32>(1, 2, 3);
     let b = 4;

test/expressions/binary/mul/vec3-scalar/i32.wgsl.expected.wgsl

@@ -1,4 +1,4 @@
-[[stage(compute), workgroup_size(1)]]
+@stage(compute) @workgroup_size(1)
 fn f() {
   let a = vec3<i32>(1, 2, 3);
   let b = 4;

test/expressions/binary/mul/vec3-scalar/u32.wgsl

@@ -1,4 +1,4 @@
-[[stage(compute), workgroup_size(1)]]
+@stage(compute) @workgroup_size(1)
 fn f() {
     let a = vec3<u32>(1u, 2u, 3u);
     let b = 4u;

test/expressions/binary/mul/vec3-scalar/u32.wgsl.expected.wgsl

@@ -1,4 +1,4 @@
-[[stage(compute), workgroup_size(1)]]
+@stage(compute) @workgroup_size(1)
 fn f() {
   let a = vec3<u32>(1u, 2u, 3u);
   let b = 4u;