Sokoban: Add world3-015 to world3-019

PR: https://github.com/DoctorLai/sokoban-web/pull/31

world3-015

https://doctorlai.github.io/sokoban-web/?level=world3-015

world3-016

https://doctorlai.github.io/sokoban-web/?level=world3-016

world3-017

https://doctorlai.github.io/sokoban-web/?level=world3-017

world3-018

https://doctorlai.github.io/sokoban-web/?level=world3-018

world3-019

https://doctorlai.github.io/sokoban-web/?level=world3-019

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