Add support for JS debugging in Fantom (#53215)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/53215

Changelog: [internal]

This adds a new environment variable to Fantom that allows debugging the JS code in tests.

Usage:

```
FANTOM_DEBUG_JS=1 yarn fantom <test>
```

**Does NOT work in OSS yet**. We need to include a third-party library to send HTTP and WebSocket requests and implement a wrapper on top of it.

Reviewed By: christophpurrer

Differential Revision: D79883372

fbshipit-source-id: d077c373a036033344e61d58274d5cd14028bda4