Use separate options for enum values

Having support for `enum` flags is great! But sometimes you don't want the user to use the actual enum values. In fact, they could easily be re-factored causing breaking changes for existing command line integrations.

It would be nice if you could specify names which would then be used instead of the enum values. E.g. using the [DisplayAttribute](https://docs.microsoft.com/en-us/dotnet/api/system.componentmodel.dataannotations.displayattribute?view=net-6.0).

```c#
[Flags]
public enum MyEnum
{
None = 0,
[Display(Name = "a")]
VeryLongEnumValueNameHere = 1,
[Display(Name = "b")]
EvenLongerEnumValueNameHereWithNoRiskForTypos = 2,
[Display(Name = "c")]
WouldBeNiceWithShorterName = 4
}
```

The corresponding command line could look like this: `foo.exe --my-enum-value a|b|c`

Or even better...

**Allow OptionAttribute on enum values?**
We could allow the `OptionAttribute` on enum values and get separate arguments for each enum flag:
`foo.exe -a -b -c`

```c#
[Flags]
public enum MyEnum
{
None = 0,
[Option('a', "long-value", HelpText = "Long value")]
VeryLongEnumValueNameHere = 1,
[Option('b', "even-longer-value", HelpText = "Even longer value")]
EvenLongerEnumValueNameHereWithNoRiskForTypos = 2,
[Option('c', "nice", HelpText = "Would be nice")]
WouldBeNiceWithShorterName = 4
}
```

Feasible?

まず、enum flags がどのように解析され、OptionAttribute のメタデータがどのように処理されるかを追跡します。提案されている DisplayAttribute 名と、enum の値に OptionAttribute を許可する方法を比較し、そのうえで既存の command-line の動作に適合するアプローチを判断します。完了の条件は、サポートする design を 1 つ合意し、実装およびドキュメント化し、例の flag 値をカバーすることです。

索引モデルが issue の本文から書いたものです。