Spec: make sharedStorage functions not available in the top-level worklet script environment

[alexmturner]

This is currently implemented but not in the spec, but top-level accesses to sharedStorage should fail in worklet global scopes, e.g.:

class Operation {
  async run(data) {
    // ok to access sharedStorage here
  }
}
register("operation", Operation)

sharedStorage.get(...)  // This should fail