Add code completion support to ::load, :load, and :pref

src/main/java/de/prob2/jupyter/commands/CommandUtils.java

src/main/java/de/prob2/jupyter/commands/LoadCellCommand.java

src/main/java/de/prob2/jupyter/commands/LoadFileCommand.java

src/main/java/de/prob2/jupyter/commands/PrefCommand.java