creating ch13.js file
replacing print with console.log
adding a link to link between javascript.html and ch13.js file
adding a function to unregister the event
adding a function that shows an event
making a function that reports a click made by user
unregistering a report click
creating a function that normalises the events
adding a handler adder function
adding a square declaration
creating example 13.2 to work with square created before
creating a function that moves a player
declaring a sokobanGame variable
creating a handler to handle the keys
creating an example 13.5 to clear content for square variable