Make it possible to make commands non-optional

[lecode-official]

In some scenarios, the user has to choose one of the possible commands, a simple example for this is the dotnet add command, which on its own does not make any sense. In this case there are two different sub-commands, which are non-optional: package and reference. For example both dotnet add package and dotnet add reference are valid command line arguments, but just dotnet add is invalid. It should be possible to declare non-optional sub-commands. When the sub-command is missing, an exception should be raised during parsing.