fix: Add string type to DocumentSnapshot.get

[ChromeQ]

Description

The comment above this change even gives an example of a dot notation string.

Taking string type from @google-cloud/firestore types: https://github.com/googleapis/nodejs-firestore/blob/22ac90f0a043e493f89834604bd249f69c5dc036/types/firestore.d.ts#L1587

Also the same on firebase-js-sdk:
https://github.com/firebase/firebase-js-sdk/blob/991fa271c867d59e2bed44c69c0512fdeb54bbb4/packages/firestore/src/lite-api/snapshot.ts#L359

Related issues

Fixes #7592

Release Summary

Fix the typing for DocumentSnapshot.get to allow dot notation object values

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • Android
  • iOS
• 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
  • No

Test Plan

---

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Name	Status	Preview	Comments	Updated (UTC)
react-native-firebase	❌ Failed (Inspect)			Jan 27, 2024 3:14am

[codecov]

Codecov Report

Merging #7593 (afe3b94) into main (970756d) will decrease coverage by 14.88%.
The diff coverage is n/a.

Additional details and impacted files

@@              Coverage Diff              @@
##               main    #7593       +/-   ##
=============================================
- Coverage     68.20%   53.31%   -14.88%     
- Complexity        0      742      +742     
=============================================
  Files           149      251      +102     
  Lines          5933    12420     +6487     
  Branches       1248     1938      +690     
=============================================
+ Hits           4046     6621     +2575     
- Misses         1790     5449     +3659     
- Partials         97      350      +253     

[mikehardy]

Looks good - thank you again 🙌