Move texture_storage access into type

Instead of having it as a decoration.
The old style is now deprecated and will be removed soon.

See  https://github.com/gpuweb/gpuweb/pull/1735 for the WGSL spec change.

Bug: tint:846
Change-Id: Id2fa681ddf7b97cd3fa41d7b5538029d96db7e28
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/53082
Reviewed-by: Corentin Wallez <cwallez@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>