Exclusivity isn't adjustable for Boolean flags with flag inversion settings

```
struct Options: ParsableCommand {
  @Flag(inversion: .prefixedNo)
  var x: Bool
}
options = Options.parseOrExit()
print(options.x)
```

```
$ options --x --no-x
false
$ options --no-x --x
true
```

Implicitly, they behave as if one had passed `FlagExclusivity.chooseLast`, but this is not adjustable.