Svelte exposes a
readable function to create writable stores.
Readable accepts two arguments. The first is the default value for the store. The second a start function to keep the value up to date.
When a component subscribes to the store, it will execute the start function. We'll listen for media query changes here.
The start function must return a stop function. The stop function will be executed when no more components are subscribed to the store.
The current store value can be retrieved with