Add playground benchmark (#51259)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/51259

Changelog: [internal]

We recently added `Playground-itest` as a quick way to test things with Fantom without committing them.

This does the same for benchmarks, so we can quickly answer questions like:

> is `key in obj` faster than `obj[key]`?

Without having to create a new benchmark manually.

Reviewed By: yungsters

Differential Revision: D74578297

fbshipit-source-id: d86604d459f15652d0c2e1ad16a99d011a1324ca