Show help for default command (#953)

Shows help if the default command was called without any arguments but has required arguments.
This way it does not hinder execution of a default command without any required arguments.

When called without args:
- Default command with required params -> Show help
- Default command with required params and additional commands -> Show help including additional commands
- Default command without required params -> Execute command

test/Spectre.Console.Cli.Tests/Expectations/Help/Greeter_Default.Output.verified.txt

@@ -0,0 +1 @@
+Hello World