TypeScript support for useFunctionsEmulator

[ifiokjr]

According to this comment this is now supported in the library.

This tiny PR makes it available for TypeScript users without d.ts hacks.

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[ifiokjr]

I've signed it.

[googlebot]

CLAs look good, thanks!

[Feiyang1]

Awesome! LGTM, thanks for the contribution!