Adding n latest_installed / n which latest_installed to get latest locally installed versions

[techiev2]

Pull Request

Adding n latest_installed / n which latest_installed to get latest locally installed versions

Problem

n latest resolves against remote and does not allow for local latest versions to
be used with a shortcut. 1

Solution

There are existing functions that give the latest installed local versions but is not exposed via the command line. This change adds a couple of helper functions that use the existing list version function footprints to get the latest installed NodeJS version/binary path.

ChangeLog

[shadowspawn]

On a related note, there have been issues suggesting version selection with no internet: #377 #404

This PR is headed in that direction, but won't cope with possible situations. (Try installing rc/latest for example.)

For syntax, the idea for a more general solution that comes to mind is an option flag, perhaps "local" or "cached" or "offline". n ls has a local context so makes sense to list a major version number, but might be bit ambiguous for latest.

n ls latest
n --offline latest
n ls 8
n --offline 8

I suggest opening an issue if you want to discuss this further, or proposing a particular aspect (like adding new label for latest_installed) to see if there is interest from others.

Thank you for your contributions.