Better support `ng new` use cases

# 🚀 Feature request

/cc @mgechev

### Command (mark with an `x`)
- [X] new
- [X] generate

### Description

We should rethink some of the DX around `ng new`, as it was originally intended to make new Angular applications, but has expanded somewhat, particularly with monorepos. There are three things in particular which are a bit awkward:

1. There is no way to create an Angular library (using the CLI) without using a monorepo structure. This may not be desired and makes library authorship a little more awkward.
1. To use a monorepo structure, users are supposed to use `ng new --create-application=false`, which is a pretty awkward syntax and doesn't make clear that this is intended for a monorepo/multi-app workspace.
1. Reevaluate the definition of "workspace". I can't speak for others, but I always interpreted a "workspace" as effectively a monorepository for Angular apps. Looking through [docs](https://angular.io/guide/file-structure), it seems that anything with an `angular.json` file is technically a "workspace", so `ng new` technically creates a workspace, even though it isn't a monorepo. I think this definition of "workspace" only really applies internally so we should think more critically about the language here.

Additional context: https://twitter.com/justinfagnani/status/1373336274384293889

### Describe the solution you'd like

I'm thinking we could add an extra flag to `ng new` to decide whether to make a standalone application (`ng new --type app`), a multi-app workspace (`ng new --type workspace`, equivalent to today's `ng new --create-application=false`), or a standalone library (`ng new --type library`). Note that `ng new --type app` and `ng new --type library` both *technically* create a "workspace" per the above definition. We might want to either tweak the definition of "workspace" to mean "a multi-app Angular repo" or use something like `ng new --type empty-workspace`.

This would be distinguished from `ng generate` because `ng new` makes a new repository while `ng generate` works within an existing repository. Arguably we should merge `ng new` and `ng generate` (maybe inferring from file path context whether a new repository is required).

Some of the broader questions we should discuss:

1. How should we position Angular "workspaces"? Are they for monorepositories or not?
2. How should `ng new` and `ng generate` work together or be merged?

### Describe alternatives you've considered

Apparently you can use `ng-packagr` to generate a library without a workspace, but you're losing a lot of the benefits of the CLI by doing so: https://twitter.com/Splaktar/status/1373373386802479105

Commencez par examiner le comportement existant de `ng new` et `ng generate`, ainsi que la documentation Angular liée sur la structure des fichiers d’un workspace. Comparez le flux actuel de `--create-application=false` avec les cas d’utilisation demandés pour les applications, les bibliothèques et les applications multiples. La tâche ne sera considérée comme terminée qu’une fois qu’une orientation commune aura été définie pour la terminologie des workspaces et les limites entre les commandes, avant l’implémentation.

Rédigé par le modèle d'indexation à partir du texte de l'issue.