Addresses #7

I've tried to keep it as similar to the base project to keep the conversion process as minimal as possible.

Also verified the scripts work by running the sample scenes as described by the _Preload workflow.

Few extra changes besides converting it to a UPM package (and updating the README for the same) are:

• Updating older broken reference links in the README
  • Added a new resource for an in-depth video essay on Singletons in Unity
• Adding support for Unity 6 with backward compatibility via C# preprocessor directives
  • it's worth noting the project is technically capable of being installed upto 2020.3, not sure beyond that. Let me know if you want me to update the package.json accordingly
• Removed some of the unused imports in the scripts
• Added a basic contributing file

TODO:

• Add a change log
• Link package description with all relevant urls (change log, description, license)
• Convert samples to samples~ to allow downloading samples separately
• Add a Unity readme (with a custom editor) to show how to use the sample scenes in the project