refactor: renamed 'score' into 'rolls' to avoid confusion with the score() method.

Removed the forgotten parameter on the score() method. oupsy.
Fifth frame, added strike detection.
refactor: extracted strike detection logic into method isStrike(roll)
refactor: renamed the values used inside the rolls.reduce() to make them more obvious.
Sixth frame is a free one, as expected.
All other tests, all passing easily.