Add store to the repl

[btakita]

It would be nice to have some store examples.

[Conduitry]

It also looks like currently the REPL doesn't even compile with store: true, which would be required first. I'm not sure whether it would be best to hardcode that or to provide a control for it. We currently don't have controls for dev or cascade (which are both set to non-default values), so there is precedent, but perhaps not good precedent.

[Conduitry]

@stalkerg also raised the point that there are links to the REPL from the state management section in the docs, which attempt to run the inline examples, which do not work. I haven't looked through them yet to see which are complete enough to be usable if store: true were set.

[mspanish]

Also - for anybody using https://codesandbox.io - can the Store be used there? Using the example code from the docs for importing you obviously can't find the store.js - but how to reference the store in that case?

import { Store } from 'svelte/store.js';