WIP: Implement REPL with non-blocking mainloop

This MR implements a non-blocking REPL using Node's internal readline to provide ANSI key parsing and terminal manipulation. It uses Gio UnixStreams on supported platforms and falls back to our previous gets implementation on unsupported platforms (export GJS_REPL_NO_MAINLOOP=true is provided to test the fallback)

Prior Art

• Node.js readline
• iPython (the interface REPL Python uses)

Demo

Changelist

• Implement a basic Repl using Gio Unix streams and Node's readline utilities
• Include a fallback on blocking reads for Windows and other unsupported platforms
• Replace Console.interact() with new Repl().run()
• (temporarily) add a -i flag to start the Repl specifically.

Closes #76