`_id` instead of `id` on firestore plugin

[Onurbon]

How do we feel about using _id instead of id?
Motivations:

• it would indicate more clearly that _id is a special thing introduced (and managed) by the firestore plugin
• it would feel more consistent with _fetching and _notFound
• if users also want to use they own alternative notion of id, they can just have both in the same object.

[coffee-cup]

I am personally not a fan of underscore properties, but not strongly against it. I just think it is a bit unnecessary.