feat: Support preconditions in `set()`

There is a bit of a gap in functionality between the `update()` and `set()` APIs.
- With `set()`, we can control merging or full replacement of the document, but cannot control preconditions.
- With `update()` we can control preconditions, but cannot control merging (it always does a shallow merge).

However, what if we want to to a complete replacement of the document (i.e. `set()` without merge) but enforce a precondition that the document must already exist?

If one makes extensive use of `undefined` values and wants a `exists: true` precondition, then one has no other choice but to use `update()` and then must be very careful to replace all instances of `undefined` with `FieldValue.delete()`.

I would propose including the `lastUpdateTime` and `exists` precondition fields in the `SetOptions` so that we can apply preconditions on a set operation, e.g.:

```ts
export type SetOptions =
| ({ merge?: boolean } & Precondition)
| ({ mergeFields?: Array } & Precondition);
```

Subsequently, a minor win of then using `set(..., { exists: true })` instead of `update(...)` is saving the bandwidth and overhead of calculating and transmitting the field mask with the request to the backend.