Breaking: update return types for fetch and write actions.

breaking changes

fetch

fetch() now returns the fetched data immediately:

Before:

const myModule = await magnetar.collection('my-collection').doc('my-doc').fetch()
const data = myModule.data

After:

const data = await magnetar.collection('my-collection').doc('my-doc').fetch()

// or

const myModule = magnetar.collection('my-collection').doc('my-doc')
await myModule.fetch()
const data = myModule.data

merge, assign, replace, deleteProp

Before:

const myModule = await magnetar.collection('my-collection').doc('my-doc').merge({ key: 'value' })

myModule // the same `doc('my-doc')` ref

After:

const newData = await magnetar.collection('my-collection').doc('my-doc').merge({ key: 'value' })

newData // the modified data after `merge()`

delete

Before:

const myModule = await magnetar.collection('my-collection').doc('my-doc').delete()

myModule // the same `doc('my-doc')` ref

After:

const result = await magnetar.collection('my-collection').doc('my-doc').merge({ key: 'value' })

result // undefined