Automate publishing

[trevjonez]

While publishing rc06 I may have forgot to pull locally before doing so. The result was an incorrect artifact being pushed to bintray/jcenter for the composer/commander jars. I overwrote the incorrect artifacts as quickly as I could, but such a mistake would be impossible had I been using an automated process driven by the github gui. My apologies if you somehow managed to cache the bad jar files in the 5 minutes between the error and resolution

todo list for myself:

• [x] RTFM for github actions syntax
• [x] RTFM for github actions secrets setup
• [x] Combine the that new found knowledge to automate publishing via the "new release" gui.

[trevjonez]

https://github.com/trevjonez/composer-gradle-plugin/actions/runs/228200752