Add JavaRunner plugin type, for executing classes

This will give us extensibility with respect to how the Java script
language runs Java code, rather than always just launching main.
Add a JavaService for managing JavaRunner plugins

The Java service could potentially do other stuff in the future too, but
mainly we need a run(Class<?>) method for executing Java code right now.
Add a JavaRunner that simply calls the main method
Add a JavaRunner for Command classes
JavaEngine: use the JavaService to run the class