Refactor code completion for commands with multiple arguments

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

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

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

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

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