publish as a Unity standard github package

[tribbloid]

This allows the package to be imported directly in Unity Editor / package manager.

at this moment, importing it will cause the following error:

[Package Manager Window] Error adding package: https://github.com/jinincarnate/off-screen-indicator.git.
Unable to add package [https://github.com/jinincarnate/off-screen-indicator.git]:
  [https://github.com/jinincarnate/off-screen-indicator.git] does not point to a valid package. No package manifest was found.

As a trade-off, package manifest and all asset directories will need to be visible at root level