Mutate `$type->isFalse()->yes()` into `$type->toBoolean()->isFalse()->yes()`

[staabm]

we could have a $type->isFalse()->yes()/$type->isTrue()->yes() to $type->toBoolean()->isFalse()->yes()/$type->toBoolean()->isTrue()->yes() mutator.

it would make us aware of false/falsey coverage.

idea from phpstan/phpstan-src#4473 (comment)