cli: extend control over defaults

[ttytm]

The PR extends functionality of the cli module to allow more control over defaults.

Creating an application with the cli module and default commands and will result in something like:

Usage: <cool_upcoming_app> [flags] [commands] <paths>

Flags:
  -i  --include       Lorem ipsum dolor.
  -h  --help          Prints help information.
  -v  --version       Prints version information.
      --man           Prints the auto-generated manpage.

Commands:
  help                Prints help information.
  version             Prints version information.
  man                 Prints the auto-generated manpage.

An example result with more direct control over defaults could look like (keeps some default flags):

Usage: <cool_upcoming_app> [flags] <paths>

Flags:
  -i  --include       Lorem ipsum dolor.
  -h  --help          Prints help information.
  -v  --version       Prints version information.

Current customization possibilities are limited as they only disable flag and command at once.

mut app := cli.Command{
	// ...
	disable_man: true
	disable_help: true
	disable_version: true
}

The updated functionality would allow full control

mut app := cli.Command{
	// ...
	defaults: struct{
		man: false
		help: cli.CommandFlag{false,true} // Fine grained control is also possible.
		version: cli.CommandFlag{false,true}
	}

}

The current disable_man, disable_help, disable_version flags are deprecated but will continue to work.

I had this potential change sitting since October but didn't submit it because there were some issues regarding its implementation. After the recent development and since private fields are now actually treated as private to a module, this change can work out. Also, I find myself in the situation again where I'd like to be able to easily disable default flags or commands but not both when working with the cli module. So I'd like to give it a shot.

Would just need to be refined and covered with a test.

[ttytm]

If DefaultOptions or something would be better fitting - the name is open for suggestions.

[spytheman]

A exhaustive type match like this match cmd.defaults.help { bool {...} CommandFlag {} }, is usually preferable, since if we later change the sumtype to type Defaults = CommandFlag | bool | string, the code will break in exactly the places, that need modification.