Add support for a decentralized templating system with `spin new`

[jaydrogers]

Discussed in https://github.com/serversideup/spin/discussions/57

^{Originally posted by jaydrogers January 8, 2024}

👉 Describe the problem

• All templates for Spin must be committed to the repository, meaning it's difficult to scale from a community perspective
• People want to contribute templates, but they don't know how

👥 Problem evidence & reach

YouTube comment (https://www.youtube.com/watch?v=I_dq-kRDztI):

🥰 Describe the "impact" on users?

This would make the community be more self-sustained and improve compatibility for other people to start using Spin.

🏆 How to solve this problem

Build a template system based off of NPM

We can use the existing package delivery system of NPM to distribute Docker compose templates. Yeah, I know -- I can't believe I just said that.

Hear me out...

• NPM is a package system that people already use and most are familiar with
• Users can easily create their own templates if needed
• Users can publish their own templates and share it with the world `spin new @myorg/my-awesome-template

Have support for "official templates"

We will continue to support official templates where users can run spin new laravel which would just shortcut to a Server Side Up package.

Guides & Documentation

• [ ] Create a developer guide on how to build and test their own templates