feat: add bonus levels

[joneugster]

Add an option that worlds can be marked as completed before finishing all levels.

• In the level files (.lean) add a command FinalLevel which when added after Level xy marks the last compulory level.
• Mark bonus levels pastel golden (?) in the world overview (or a bit darker golden if playable)
• Once the final level is completed:
  • unlock the next worlds
  • always display two buttons "Leave world" and "Bonus" (or the existing "Next") as long as there are still bonus levels.