Add manifest package.json to the project

[FSM1]

Summary Unity Package Manager requires a package.json to be included in the package.

Motivation This will improve the ability for developers to import the package from UPM.

Additional context See docs around the structure of the package.json here: https://docs.unity3d.com/Manual/upm-manifestPkg.html

[kalambet]

Looks like this might be a subtask for #316

[Arshia001]

Blocked until the single-repository core library is ready

[kalambet]

Fixed a while ago. See ^HEAD