A prototype inclusion of effect handling into the `Lwt_main.run` function

[raphael-proust]

Using two schedulers at the same time doesn't really work. Even when they're not really schedulers! And so it's unsurprising that effects and Lwt don't mix well. See [https://gitlab.com/raphael-proust/lwt-effect-try-out/-/blob/develop/src/M01pause.ml](this experiment) for examples of where it breaks down.

This PR is a prototype to allow users to pass an effect handler to the Lwt_main.run function which in turn installs it around the appropriate calls.