Add mutuallyExclusive combinator

[Schniz]

There's something that makes me sad regarding the current implementation: we can't do exclusive or for flags like --dev and --prod, because they have defaults. Should I make it a special case? Should we ignore default values? Just like I have added nodes to the result of a parsing, I can add fromDefaultValue: boolean, and ignore these. But then, what would the output be? of the first default value?

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