Fix typing in AsyncStoreOptions

[Beramos]

https://github.com/square/svelte-store/blob/ccb97fce941344c510f2e80eedb523a477df6213/src/async-stores/types.ts#L37C1-L41C2

I think "true" should be "boolean", the following returns an error

  const {store: positionStore, state: positionState} = asyncReadable(
    {},
    async () => {
      const response = await fetch(URL_API + 'positions');
      const userObject = await response.json();
      console.log(userObject)
      return userObject;
    },
    { reloadable: false, trackState: true }
  );

Type 'false' is not assignable to type 'true'.ts(2322)
(property) reloadable?: true

[Beramos]

I'm happy to contribute here