After the previous commit, nushell uses PrettyDebug and
PrettyDebugWithSource for our pretty-printed display output.
PrettyDebug produces a structured `pretty.rs` document rather than
writing directly into a fmt::Formatter, and types that implement
`PrettyDebug` have a convenience `display` method that produces a string
(to be used in situations where `Display` is needed for compatibility
with other traits, or where simple rendering is appropriate).