add support for unity package manager

[unitycoder]

so that can install plugin directly from Unity Package Manager

references:

• https://gist.github.com/LotteMakesStuff/6e02e0ea303030517a071a1c81eb016e
• https://nagachiang.github.io/tutorial-working-with-custom-package-in-unity-2019-2/
• https://medium.com/openupm/how-to-maintain-upm-package-part-1-7b4daf88d4c4