Proposal: export firebase package to reduce required imports

[SupposedlySam]

Suggested Change

1. Include firebase_core in dependencies for firefuel
2. Export firebase_core and cloud_firestore from firefuel
3. Update docs/readme with directions on how to override versions of the above deps if needed

Pros

• Easier to use firefuel
• Less dependencies the user needs to manage

Cons

• Users will need to know to import firefuel rather than firebase_core or cloud_firestore
• There may be more confusion about where the responsibilities of the firefuel package start and end
  • This could lead to a lot of redirecting to the other packages when issues come our way
• We will have to be careful to never overlap any names from the above packages