[Proposal] Moving files & packages to aurelia-cli

Hey everyone ! 👋

After the default installation of `Aurelia` with the CLI somes files are created under `aurelia_project`.

Those are Gulp's tasks, stubs for generators, environment configuration and the `aurelia.json`.

I think that those files shouldn't be managed by the developer.
All gulp packages and tasks can be moved inside the CLI and same goes for stubs.

This will allow the aurelia team to manage them easily.
Actually, we (devs) need to check with a new installation if tasks has been changed, gulp package updated, etc.

I feel that the current default installation of aurelia with the CLI is kinda bloated cause of that. We will remove 12 packages of the `package.json`, it will also ensure that aurelia provide the version of package it supports (https://github.com/aurelia/cli/issues/636).

So to resume, my proposal is to...

1. Move all Gulp's packages to `aurelia/cli` directly.
2. Move all stubs to `aurelia/cli` directly.
3. Move all Gulp's tasks to `aurelia/cli` directly.
4. Move env's configuration to `aurelia.json`.
5. Move `aurelia.json` to the root of the project.
6. Create a command to "extract" tasks and stubs to be able to custom them (like React `eject` command).

What do you think?

Review the aurelia_project contents, aurelia.json, package.json, Gulp packages and tasks described in the proposal, then trace their handling through the CLI entry points. Confirm the intended project layout and extraction workflow with maintainers; done means the migration and customization behavior are specified, implemented, and covered by tests.

Written by the indexing model from the issue text.