Show allowed values in help text for Nullable/ValueTuple<bool,T> option/argument

[scott-xu]

Fix #388

[scott-xu]

@natemcmaster
If the option type is Nullable<T>, should we return CommandOptionType.SingleOrNoValue if T is CommandOptionType.SingleValue
https://github.com/natemcmaster/CommandLineUtils/blob/main/src/CommandLineUtils/Internal/CommandOptionTypeMapper.cs#L63

[scott-xu]

@natemcmaster
If the option type is Nullable<T>, should we return CommandOptionType.SingleOrNoValue if T is CommandOptionType.SingleValue
https://github.com/natemcmaster/CommandLineUtils/blob/main/src/CommandLineUtils/Internal/CommandOptionTypeMapper.cs#L63

I ask because I have to specify CommandOptionType.SingleOrNoValue. Otherwise, the help text would be

  -enumOpt3|--verb3 <VERB3>         nullable enum option desc.
                                    Allowed values are: None, Normal, Extreme.

rather than

  -enumOpt3|--verb3[:<VERB3>]       nullable enum option desc.
                                    Allowed values are: None, Normal, Extreme.

CommandLineUtils/test/CommandLineUtils.Tests/DefaultHelpTextGeneratorTests.cs

Lines 208 to 209 in b10e802

	[Option(CommandOptionType.SingleOrNoValue, ShortName = "enumOpt3", Description = "nullable enum option desc.")]
	public SomeEnum? Verb3 { get; set; }

[natemcmaster]

Looks good to me! There is a merge conflict now, though, with your other PR. Can you resolved and update?

[natemcmaster]

re: your question

If the option type is Nullable, should we return CommandOptionType.SingleOrNoValue if T is CommandOptionType.SingleValue

Let's open a separate issue for this. We need to think through backwards compatibility of this change given that the current behavior is SingleValue. I think changing to SingleOrNoValue could break some users unless we also improve the parser to support --a value for SingleOrNoValue.

[scott-xu]

Conflicts are resolved.
Issue #394 is created.

[natemcmaster]

Thanks @scott-xu !