Upgrade to Firebase v8

[JFGHT]

Steps to reproduce the behavior

Simply use vuefire/vuexfire with Firebase v8.

Expected behavior

Everything working fine.

Actual behavior

Error in many files (types).

Error in node_modules/@posva/vuefire-core/dist/packages/@posva/vuefire-core/src/firestore/index.d.ts(3,10):
Module '"../../../../../../../../firebase"' has no exported member 'firestore'. Did you mean to use 'import firestore from "../../../../../../../../firebase"' instead?

3 | import { firestore } from 'firebase';

[posva]

Duplicate of #940

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