FR: Expose DocumentReference.create on Web

I would like to create a document from the browser using behaviour similar to `O_CREAT | O_EXCL`. [8 other people are also looking for a clean way to do this](https://stackoverflow.com/q/59989973/10477326). This functionality is present in the REST API, it's just not exposed to the JavaScript SDK.

This differs from #1795 in that I'm not using any testing or admin SDK. They found a workaround. I also found a workaround, but I am creating this feature request because that workaround feels like a code smell.

### [REQUIRED] Describe your environment

* Operating System version: (documentation; I haven't downloaded any code yet)
* Browser version: (documentation)
* Firebase SDK version: Javascript - version 9 and Javascript - version 8
* Firebase Product: Firestore

### [REQUIRED] Describe the problem

- The underlying REST API supports this: https://firebase.google.com/docs/firestore/reference/rest/v1/projects.databases.documents/createDocument
- The NodeJS API supports this: https://googleapis.dev/nodejs/firestore/latest/DocumentReference.html#create
- 6 people are confused why only the Web API lacks this: https://stackoverflow.com/questions/46597327/difference-between-firestore-set-with-merge-true-and-update#comment80153511_46600599
- I do not like the workaround that relies on security rules
- I do not like just ignoring this and having to worry about the potential of a race condition
- Whenever underlying capability exists but is hidden, a workaround is not the best solution. Furthermore, the 4 perfect letters of CRUD are cut short into just 3 letters RUD.

#### Steps to reproduce:

1. Open https://firebase.google.com/docs/reference/js/firestore_.documentreference
2. https://firebase.google.com/docs/reference/js/v8/firebase.firestore.DocumentReference
3. CTRL-F for "create"
4. Find no actual "create" method

#### Relevant Code:

This problem is apparent at the documentation level, not the code level.