feat: introduce `@auth/firebase-adapter` (#7803)
Database adapters are not dependent on Next.js features, so it makes sense to republish them under the `@auth/*` scope.
This PR is part of a series to convert adapters, using `@auth/core` for types.
BREAKING CHANGE:
If you are coming from the previous adapter, change your `package.json`:
```diff
- "@next-auth/firebase-adapter": "0.0.0",
+ "@auth/firebase-adapter": "0.0.0",
```
And run `npm install`, `yarn install` or `pnpm install` respectively.
**Note:** This packages is published as ESM-only
