Misc fixes for firestore-exp API definition

[schmidt-sebastian]

Fixes some random issues in the index.d.ts file for the unreleased tree-shakeable clients. I also went back to the prior defintion of SetOptions, since it "just works" (tm) and allows me to write less validation code (no need to port the check that
"merge" and "mergeField" cannot be combined). Let me know if there are any concerns.

[changeset-bot]

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Latest commit: ef4c6e1

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[google-oss-bot]

Binary Size Report

Affected SDKs

• @firebase/messaging

  Type	Base (ee1892d)	Head (af2c06a)	Diff
  esm2017	23.2 kB	23.2 kB	+35 B (+0.2%)
  main	31.4 kB	31.4 kB	+35 B (+0.1%)
  module	30.9 kB	30.9 kB	+35 B (+0.1%)

• firebase

  Type	Base (ee1892d)	Head (af2c06a)	Diff
  firebase-messaging.js	39.1 kB	39.2 kB	+43 B (+0.1%)
  firebase.js	819 kB	819 kB	+43 B (+0.0%)

Test Logs

• Base (ee1892d6): https://github.com/firebase/firebase-js-sdk/actions/runs/159642330
• Head (af2c06ab): https://github.com/firebase/firebase-js-sdk/actions/runs/159858313

[thebrianchen]

LGTM