Fix: Load initial value from storage

[joshnuss]

This fixes a bug where the initial value set incorrectly. Closes #231

Previously, it only read the initial value inside store's closure:

writable(initialValue, set => {
  // read from storage *inside* closure
  const json = storage.getItem(key)
  const existingValue = JSON.parse(parsedValue)
  
  // ...
})

But that closure is only called on first get.

Instead, the initial read should happen before the store is even created:

// read from storage **outside** closure, before initializing store
const json = storage.getItem(key)
const initialOrExisting = json ? JSON.parse(json) : initialValue

writable(initialOrExisting, set => {
  // ...
})

[github-actions]

🎉 This PR is included in version 0.8.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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