Bump typescript from 3.9.7 to 4.0.2 in /functions (#60)

Bumps [typescript](https://github.com/Microsoft/TypeScript) from 3.9.7 to 4.0.2.
- [Release notes](https://github.com/Microsoft/TypeScript/releases)
- [Commits](microsoft/TypeScript@v3.9.7...v4.0.2)

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Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

functions/package-lock.json

@@ -25,7 +25,7 @@
   "devDependencies": {
     "firebase-functions-test": "^0.2.1",
     "tslint": "^6.1.2",
-    "typescript": "^3.9.7"
+    "typescript": "^4.0.2"
   },
   "private": true
 }