Use a better value for the default groupId

This project migrated from ImageJ2, where net.imagej was an intuitive
choice for default groupId. But the project is part of the Scijava
scripting framework now, so a groupId matching that fact makes more
sense.

We use org.scijava.scripting.java, rather than only org.scijava, to
avoid potential name clashes with actual SciJava projects such as
scijava-common, scripting-java, etc.