Typescript: Second argument to readable should be optional

[utherpally]

Describe the bug
I think creating a store without a start/stop notifier is totally valid for some special use cases (e.g. SSR friendly).

I write a lot of things like this in my SvelteKit(svelte@3.38.2) app to make Typescript checker happy:

function noop() {}

export const useMedia = (query: string) => {
  if(!browser) {
    return readable(false, noop); // 1
  }
  // Do something real here
}

export const useWindowSize = ({initialWidth = Infinity, initialHeight = Infinity}) => {
  if(!browser) {
    return {
      width: readable(initialWidth, noop), // 2
      height: readable(initialHeight, noop), // 3
    }; 
  }
  // Do something real here
}

Expected behavior
store = readable(neverChangedValue) etc should be accepted by TypeScript.

Sorry for my bad English

[Conduitry]

These types have been updated in 3.38.3.