`_ready` field in firestore plugin

[Onurbon]

Sometimes I want to distinguish three cases:

const data = db.things.get(id)
if (data._fetching) return <Spinner/>
if (data._notFound) return <NotFound/>
return <TheThing />

but sometimes I'm lazy and just want a spinner for two first cases, so it's be nice to just write

if (!data._ready) return <Spinner/>

were _ready === !_fetching && !_notFound

[coffee-cup]

I can see the use of it, however, it might it easy for users to just handle not ready the same as fetching, which is not always the case. We might want to force users to handle the case where the data is not found or there was an error.