Replace callbacks with assignment

[tjvr]

We could do a few things to make you-win easier for beginners.

Replace arrow functions () => { ... } with function literals function() { ... }, to match the JS course.

Replace player.forever(function() { ... }) with player.onTick = function() { ... }, to reduce the number of brackets you need to type, and move toward things being defined in a single place. Beginners often end up with multiple forever loops and I think this is a point of confusion.

On that note: I think the distinction between initialization (at the top-level), and doing things every frame (inside a forever), is not the easiest to grasp. I wonder if it would be better to have an explicit initialisation step, something world.onBegin = function() { ... } , or uw.onBegin = function .... This would roughly match Scratch's "when green flag clicked".

I worry that our current style relies far too much on scope, e.g. creating a makeBullet() function. I wonder if it would be better to move towards a more class-based/OOP approach, like we did in the old Python Games course...

As a minor point, if it's possible to configure the editor we use to auto-indent / pretty-print on save, that would reduce a major proportion of the errors.