Add document ref as readonly to document

[hamishjohnson]

You can use this.$firestoreRefs to get the refs of documents.

I think it would be cleaner to use ref as a non-enumerable, read-only property like the ID. I would suggest that using the dollar syntax would avoid conflicts.

So then you can access the ref by doc.$ref

[posva]

Duplicate of #180

[posva]

FYI this can be achieved already in userland, as commented in the issue