Adds a continuous integration workflow to the repository, using the [Forgejo Actions](https://forgejo.org/docs/next/user/actions/) framework.
Concurrently, Aaron added a [wiki page](https://code.studioinfinity.org/glen/dyna3/wiki/Continuous-integration) to document the continuous integration system. In particular, this page explains how to [run continuous integration checks on a development machine](wiki/Continuous-integration#execution), either directly or in a container.
Co-authored-by: Aaron Fenyes <aaron.fenyes@fareycircles.ooo>
Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Reviewed-on: StudioInfinity/dyna3#75
Co-authored-by: Vectornaut <vectornaut@nobody@nowhere.net>
Co-committed-by: Vectornaut <vectornaut@nobody@nowhere.net>
