add an empty typings file to firebase

[Feiyang1]

Hopefully to address the auto-completion issue with Webstorm - #5649

[changeset-bot]

⚠️ No Changeset found

Latest commit: c055120

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[google-oss-bot]

Binary Size Report

Affected SDKs

No changes between base commit (317394d) and head commit (9cacf84).

Test Logs

• Base (317394d2): https://github.com/firebase/firebase-js-sdk/actions/runs/1391524369
• Head (9cacf84e): https://github.com/firebase/firebase-js-sdk/actions/runs/1391813933

[google-oss-bot]

Size Analysis Report

Affected Products

No changes between base commit (317394d) and head commit (9cacf84).

Test Logs

• Base (317394d): https://github.com/firebase/firebase-js-sdk/actions/runs/1391524369
• Head (9cacf84): https://github.com/firebase/firebase-js-sdk/actions/runs/1391813933