Consider publishing zellige-core maven artifact

In addition to letting people re-use zellige as a library in Maven-based projects, publishing a maven artifact including a correct POM would allow the wiki page on https://imagej.net/plugins/zellige to correctly render its infobox.

The CI build script provided by scijava-scripts should also support GitLab CI: https://github.com/scijava/scijava-scripts/blob/22675f34ab95dae404d848abcc3351d0f2946af1/ci-build.sh

(But setup of CI for deployment, including adding the required secrets, needs input from Curtis Rueden or Mark Hiner.)