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/shader_io/fragment_output_builtins.wgsl b/test/shader_io/fragment_output_builtins.wgsl
index 462cb77..914cbad 100644
--- a/test/shader_io/fragment_output_builtins.wgsl
+++ b/test/shader_io/fragment_output_builtins.wgsl
@@ -1,9 +1,9 @@
-[[stage(fragment)]]
-fn main1() -> [[builtin(frag_depth)]] f32 {
+@stage(fragment)
+fn main1() -> @builtin(frag_depth) f32 {
   return 1.0;
 }
 
-[[stage(fragment)]]
-fn main2() -> [[builtin(sample_mask)]] u32 {
+@stage(fragment)
+fn main2() -> @builtin(sample_mask) u32 {
   return 1u;
 }