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?

先追蹤 enum flags 的解析方式，以及 OptionAttribute 中繼資料的處理方式。比較提議的 DisplayAttribute 名稱與允許在 enum 值上使用 OptionAttribute 的做法，然後判斷哪種方法符合現有的 command-line 行為。完成的條件是已達成共識、實作並記錄一種受支援的 design，且涵蓋範例中的 flag 值。

由索引模型根據 Issue 內容生成。