Enable client calls to be made from the REPL

[dhil]

Currently, it is not possible to invoke a client function, whilst inside the REPL. There are multiple ways to support this functionality. One possible approach is to spin a Node instance up on the side and communicate with it using the native node API. The prerequisites for doing so are at least:

• [ ] Implement OCaml bindings for the native node API.
• [ ] Separate the client runtime into web browser dependent components and non-web browser dependent components.
• [ ] Simulate the DOM via some virtual DOM library.

I reckon implementing this facility could make for one or more good student projects.