Add `PrettyPrint` library

[ssoelvsten]

In the long-term, we would like to implement the Troupe compiler in Troupe itself. To this end, we'd need a pretty printing library similar to Text.PrettyPrint.HughesPJ we currently use in the Haskell-based compiler. One might want to take inspiration from tkadur/prettyprinter-sml or other ML-based pretty printers for an ergonomic syntax.

This can be used for other things too, so we may as well add it to the standard library (lib/). Furthermore, it is a good way to stress test Troupe's string operations.