Support Async Storage interface in ExternalsOptions

[unadlib]

I'm getting a type error when passing a localforage instance to the localStorage property in ExternalsOptions.

Type 'LocalForage' is not assignable to type 'Storage'.
  Types of property 'length' are incompatible.
    Type '(callback?: ((err: any, numberOfKeys: number) => void) | undefined) => Promise<number>' is not assignable to type 'number'.ts(2322)

This is likely because the ExternalsOptions type is not currently compatible with the asynchronous storage interface. The relevant code is as follows:

export interface ExternalsOptions {
    fetch?: typeof fetch;
    Request?: typeof Request;
    Response?: typeof Response;
    Headers?: typeof Headers;
    localStorage?: Storage;
}