`nix repl`: Group of `:?` by use case

Problem

When you type :? in nix repl you are presented with a list of 17 items, mostly sorted alphabetically by command name.

Due to people's limited ability to account more than ~5 objects of interest at a glance, it's improbable for beginners to find what they care about, and it's similarly improbable for most people to read through the list carefully.

Proposal

In order to make the available commands easier to discover and understand how they relate to each other, group the output of :? (source) by use case, for example:

• evaluate
• build
• inspect
• quit

Checklist

• checked latest Nix manual (source)
• checked open documentation issues and pull requests for possible duplicates

Priorities

Add :+1: to issues you find important.