[WGSL] Support Synchronization Built-in Functions - https://www.w3.org/TR/WGSL/#sync-builtin-functions

First two are easy: [WGSL] => [MSL] mapping
* storageBarrier => threadgroup_barrier(mem_flags::mem_device)
* workgroupBarrier => threadgroup_barrier(mem_flags::mem_threadgroup)

Last one expands to this I think
* workgroupUniformLoad => 
uint metalWorkgroupUniformLoad(threadgroup T* const p) 
{
  threadgroup_barrier(mem_flags::mem_threadgroup);
  const T result = *p;
  threadgroup_barrier(mem_flags::mem_threadgroup);
  return result;
}