Expose DeepReadonly<> type

[stefnotch]

What problem does this feature solve?

Currently it's not possible to create an accurate interface for a composition function that returns a readonly(someObject).

For example

interface QuantumDocument  {
  documentState: ???
}

function useQuantumDocument(): QuantumDocument {
  const documentState = reactive({ });
  // code code code
  return {
    documentState: readonly(documentState) // return a readonly version of it
  }
}

What does the proposed API look like?

I propose simply exposing the DeepReadonly<T> type.