Convert the default init script to an extension

[andrus]

There is a file jjava-jshell-init.jshell that loads the following script in each notebook. Let's switch that to an extension mechanism. This way, we can disable loading anything but the bare Java environment (see #38). This will help with issues like SpencerPark/IJava#156.

import java.util.*;
import java.io.*;
import java.math.*;
import java.net.*;
import java.util.concurrent.*;
import java.util.prefs.*;
import java.util.regex.*;

import static org.dflib.jjava.runtime.Display.*;
import static org.dflib.jjava.runtime.Kernel.*;
import static org.dflib.jjava.runtime.Magics.*;

public void printf(String format, Object... args) {
    System.out.printf(format, args);
}