Type mismatch between string and string array to error out

Noticed at https://github.com/dotnet/format/pull/551#discussion_r379924103, if `option.Argument = new Argument(() => null)` is used and the target delegate's signature has `string[]`, it can be an error so the user knows that API is not going to work as expected.

```
myApp --myStringArrayArg val1 val2 val3
```

will only function properly at runtime with `option.Argument = new Argument(() => null)`.