FR: Method to combine two firestore WriteBatch

It'd be great if `firestore.WriteBatch` had a `merge(batch: firebase.firestore.WriteBatch)` method.

The goal is to have utility methods that can return a batch. Before committing, those batches could be merged together.

(It looks like a basic implementation will be to concat the `batch. _mutations`).

```ts
function starBlogArticle(articleId: string): firebase.firestore.WriteBatch {
const batch = firebase.firestore().batch();

batch.set(...); // Some changes
batch.update(...);

return batch;
}

function createBlogArticle(articleId: string, title: string): firebase.firestore.WriteBatch {
const batch = firebase.firestore().batch();

batch.set(...);
batch.set(...);

return batch;
}

...

// On the application side

const onClickCreate = () => {
const articleId = '...';
const batch = createBlogArticle(articleId, title);
batch.merge(starBlogArticle(articleId));

batch.commit();
}
```