Backport package.json to master branch

[favoyang]

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Hi @jeffcampbellmakesgames,

Thanks for creating the awesome package. I work on https://github.com/openupm/openupm, which is an upcoming UPM registry for open source unity package (sneak preview). The service can build and publish UPM package direcly from git repo. For unity-jtween, however certain changes are required

• backport package.json from stable branch to master, located at Unity/Assets/JCMG
• bump the version and make a new release

The openupm build pipeline will checkout each tags that is valid semver, and locate the package.json file as the root, then run npm publish from there.

An optional change is to move the Unity/Assets/JCMG folder to Unity/packages/ and rename it as com.jeffcampbellmakesgames.jtween. This way the package acts as a local package in the main unity project. Here's an example project using this layout.

The changes benefit openupm, but are good practice in general.

I also noticed the custom license, as openupm don't change any of your code, it can re-pack from source code, make tarball and distribute it. Please correct me if my understanding is incorrect.