Support for tri-state flags with [CommandFlag]

[adambajguz]

There are other CLI tools that include both options, e.g. --restore and --no-restore, where you are giving three possible values to the user: true (associated with Name), false (NoName) and default.

[CommandFlag(Name="flag", NoName = "no-flag")]
public bool? Flag{  get; init; } //bool? or custom struct Flag

Some problems:

• what about current bool flags?
• what about short flags: -abc
• what if the dafuatl is not null but true or false?