Merge: Intro and use `prompt`, an alternative to `readline`

The new package `prompt` offers two basic services to create a shell interface. It does not depend on external libraries and it is licensed under Apache 2.0.

The `prompt` API is optionally implemented by the `readline` package which can be used as a drop-in replacement.

This PR uses `prompt` in nitx to remove the dependency on readline. This dependency would prevent us from compiling Nit on platforms that do not offer the native library, and someone could argue that there are possible legal ramifications in which I would not want to ensnare the Nit project. See the past discussion on GNU readline: #2083

Anyone can still mixin readline with nitx using: `nitc nitx.nit -m readline`

This will also fix the Nit Docker image failing to build: https://hub.docker.com/r/nitlang/nit/builds/

Pull-Request: #2613