Print the help message from subcommand

The makeUsage command is `private` it should be `protected` so that sub classes can call it.

My usecase is that I have a subcommand that itself has subcommands, and if executed without a subcommand I want it to print the help message, e.g. `cli sc` prints help `cli sc ssc` does something.

My workaround is this.

```
import kotlinx.cli.*
import kotlin.reflect.full.*

class NewSubCommand: Subcommand("sc", "sc") {
override fun execute() {
// Print Help
ArgParser::class.declaredFunctions.find { it.name == "makeUsage" }?.let {
println(it.call(tiz))
}
}
}
```

Another workaround might be to throw an error with:
```
override fun execute() {
throw ParsingException("Needs Sub command")
}
```

but I think this is also not a great workaround.

Start by locating ArgParser.makeUsage and the Subcommand execution path described in the issue. Check how a subcommand with nested subcommands behaves when invoked without a child command, then make the usage method callable by subclasses and verify that `cli sc` prints help while `cli sc ssc` still executes the nested command.

Written by the indexing model from the issue text.