Removes store caching during SSR

[joshnuss]

This PR removes caching of stores during Server Side Rendering (SSR)

Problem

Caching stores server side makes state leak between requests, since the global state is shared between requests

Solution

Avoid using global state during SSR (when window is undefined)

Caveats

If the a store with the same key is instantiated twice, it creates two seperate stores.

const store1 = persisted('my-key', initial)
const store2 = persisted('my-key', initial)

// during SSR
// store1 != store2, because their is no global cache of stores

// during CSR
// store1 == store2 because their is a global cache

Closes #306

[webJose]

If event.newValue is falsy, are you not interpreting that as a value deletion? I don't remember if this NPM package deletes values or not.

[joshnuss]

event.newValue is a JSON string, so if it's falsey, it's because it's undefined.
If memory serves, it's undefined when the key was deleted in another tab.

[webJose]

I just had a question. I saw nothing obviously wrong or anything. On the contrary, it looks good and adding the extra unit tests should increase the confidence on the changes.

[joshnuss]

All good,
And thanks for the review @webJose!
Extra set of eyes is much appreciated 🙏