Add jarfile generation script for github, using actions.

Add jarfile generation script for github, using actions.
This workflows builds jars on linux, osx, and windows, combine them in a final job, which outputs the jar (zipped :\ ) which can be downloaded and published.
The workflow is set up to be run on demand for now (from the actions tab on the github repo, link is a bit hidden as you have to click on the workflow descriptions to be able to run it), but we could have it run when a release is done, and the artifact attached automatically.