GitHub Action to build plugin

[philpax]

DIP17 says that there should be an action by default on this repo so that people who use it as a template will get CI for free.

[philpax]

@NotNite would you be interested in tackling this? Don't worry about making a 'generic' action, just needs to build this repo and we can figure out where to go from there

[NotNite]

Uhhh... sure - once I'm not fighting COVID.