feat(core): add snapshot cache

[illuminist]

Description

Add snapshot cache

 Get DocumentSnapshot and QuerySnapshot with object from either data or
 ordered firestore state. If provided with doc data, it will return
 DocumentSnapshot, providing with a collection from data or an array from
 ordered state will return QuerySnapshot, except ordered state that generated
 as DocumentRef will return DocumentSnapshot
 Note: the cache is local and, not persistance. Passing an object from initial
 state or from SSR state will yield undefined.

Check List

If not relevant to pull request, check off as complete

• All tests passing
• Docs updated with any changes or examples if applicable
• Added tests to ensure new feature(s) work properly

Relevant Issues

#203

[prescottprue]

Love this idea - let me know if I can be of any help!

[prescottprue]

Were you imagining this going into current or a v1.0.0 version? Seems like it could go into a new minor version since it doesn't seem to break anything or prevent backwards compatibility.

Looking forward to getting it in, thanks for the PR!

[illuminist]

It's still okay to add this to the current version as it isn't contain any breaking change though. Even though I myself don't have usage for this yet but it would be better to have people to try it out.