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.