flags: make enableAsyncDebugInfo dynamic for www (#34430)

As titled. This adds dev-only debugging information to Fizz / Flight
that could be used for tracking Promise's stack traces in "suspended by"
section of DevTools.

DiffTrain build for [c552618a8203866a8bf80324f6efa708d4bdb146](https://github.com/facebook/react/commit/c552618a8203866a8bf80324f6efa708d4bdb146)