[CLI] Prompt for plural of model for REST Controller generation

[kjdelisle]

Overview

As a LoopBack 4 user, I would like the routes generated in my controllers to follow the practice used in LoopBack 3, where my REST resource routes were plural versions of my model name (Foo -> /foos)

Acceptance Criteria

• Add a new prompt to the REST controller template that
  • Prompts for plural name and provides a sane default in the same way LoopBack 3 does (change the name to plural, convert from camel-case to kebab case, i.e. UsedCar to used-cars)
  • Allows editing of the value to whatever is preferred by the developer
  • Validate plural form is URL Safe (same as Model Name validation used today in LB4)
• Add test coverage
• Change JSDocs to explain the new feature (and flag)
• Update loopback.io documentation