Integrate LICENCE into plug.gd for portability.

[OctagonalHexy]

The current recommended way of installing gd-plug is to add plug.gd to your version control so that it installs with the repo its being used in, but the LICENSE file is separate meaning it has to be included manually in order to adhere to its licensing.

I recommend adding A copy of the licensing as A comment header to the plug.gd file itself. That way it can be included into any repo by itself.

[imjp94]

To be honest, I have zero knowledge on licensing, but it is the same case for all godot plugin.
There's always a .gitattributes file used to exclude root files(like project.godot, README and LICENSE) on installation.
So, my solution is "don't fix it, unless it is broken".

But I am still opened to the inclusion of license in plug.gd, if it is really affecting your works.