Provide a default message that informs the user of the `(e)`, `(enter)`, and
`(esc)` functionality.
Unfortunately, it is not possible to provide any message that is better
customised to each case, due to `inquire` requiring a `'a` lifetime on all its
inputs – this rules out providing any kind of `String` input.
The first paramter of `Editor::new` can already be used to provide more
contextual information. The second parameter is left configurable, but for now
`Editor::HELP` is used everywhere.
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