Commit d3f4f5f208ee76a22ff39c13df4d150712685f5f introduced automatic conversion to FileInfo and DirectoryInfo but failed to properly handle the conversion if the value comes from the [DefaultValue] attribute.
Using both `var (converter, stringConstructor) = GetConverter(...)` and `var (converter, _) = GetConverter(...)` should have been a red flag!
Allow custom help providers
* Version option will show in help even with a default command
* Reserve `-v` and `--version` as special Spectre.Console command line arguments (nb. breaking change for Spectre.Console users who have a default command with a settings class that uses either of these switches).
* Help writer correctly determines if trailing commands exist and whether to display them as optional or mandatory in the usage statement.
* Ability to control the number of indirect commands to display in the help text when the command itself doesn't have any examples of its own. Defaults to 5 (for backward compatibility) but can be set to any integer or zero to disable completely.
* Significant increase in unit test coverage for the help writer.
* Minor grammatical improvements to website documentation.
When subclassing `Command<TSettings>` which has the [NotNull] attributes, Rider/ReSharper gives this warning:
> Nullability of type of parameter 'context' in method does not match overridden member `int Spectre.Console.Cli.Command<TSettings>.Execute(CommandContext, TSettings)` (possibly because of nullability attributes)
When subclassing `Command<TSettings>` which does not have the [NotNull] attributes, Rider/ReSharper gives this warning:
> The nullability attribute has no effect and can be safely removed
The solution is simply to remove the [NotNull] attributes.
Since `<Nullable>enable</Nullable>` is set in the project, they are actually not necessary. By the way, the non-generic `Command` class does not have the [NotNull] attributes.
Add support for converting command parameters that doesn't have a built-in TypeConverter but has a constructor that takes a string. For CLI apps, FileInfo and DirectoryInfo will likely be the most useful ones, but there may be others.
When a conversion to an enum fails, list all the valid enum values in the error message.
Message before this commit:
> Error: heimday is not a valid value for DayOfWeek.
Message after this commit:
> Error: Failed to convert 'heimday' to DayOfWeek. Valid values are 'Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'
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
