Restructure project into UPM package

[Saucy]

Hello,

I took the liberty to restructure your project into a package that can easily be imported through Unity Package Manager (UPM).

• Added .md meta files that were missing
• Added Assembly Definition files to folders
• Updated README, gave it some headers
• Updated .gitignore based on Unity.gitignore
• Updated location of files and folders (most were already in correct folders)
• Updated scripts to UnityEngine.GUID namespace

*Note: I use Rider for Unity and Rider formatted all scripts, I have no idea how to stop it from using my own code style in other people's projects, sorry if this causes any trouble!!