setItem should return the dispatch promise

Hey there 👋

Currently there's no way to know, when a item was set to a storage (except for adding a listener, but this doesn't work for a control flow).

const myPlugin = {
  initialize: ({ instance }) => {
    instance.setItem('foo', 'bar');

    const foo = instance.getItem('foo');
    
    console.log(foo); // => undefined
  }
}

This happens, because internally instance.setItem fires a dispatch, which is basically async. The type of it is actually sync which leads to the getting the item directly afterwards being undefined.
instance.setItem here should be changed to return the dispatch promise, so it can get awaited.

Start at packages/analytics-core/src/index.js around line 778, where instance.setItem is defined and dispatch is triggered. Trace the dispatch call and its returned promise, then verify that awaiting setItem allows a subsequent getItem('foo') to observe the stored value. Done means the API returns the dispatch promise and its type reflects that asynchronous result.