chore: Allow type-safety for subcollections

[fivecar]

trafficstars

Description

Root collections currently allow strong typing:

const allYourBase = firestore()
  .collection<SetUsUp>("are-belong")
  .get();

But subcollections cannot be typed:

const allYourBase = firestore()
  .collection<SetUsUp>("are-belong")
  .doc("8675309")
  .collection<TheBomb>("to-us")  // This is not accepted by index.d.ts
  .get();

This PR attempts to make that possible.

Release Summary

Support strong typing for subcollections

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • [X] Yes
• My change supports the following platforms;
  • [X] Android
  • [X] iOS
  • [X] Other (macOS, web)
• My change includes tests;
  • [ ] e2e tests added or updated in packages/\*\*/e2e
  • [ ] jest tests added or updated in packages/\*\*/__tests__
• [ ] I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • [ ] Yes
  • [X] No

Test Plan

Installed local version in my project, and verified that Typescript accepts a subcollection type.