negateFlagValueParser function
A ValueParser that wraps another ValueParser<bool>
but flips the value.
See ValueParserFlagExtensions.negateFlag for a slightly more elegant
alternative.
var negatedParser = negateFlagValueParser(boolValueParser);
print(negatedParser('true')); // false
print(negatedParser('false')); // true
Implementation
ValueParser<bool> negateFlagValueParser(ValueParser<bool> parser) =>
parser.then((value) => !value);