Rainy Day JSRL dev branch migrated to webpack+JSRL

I just updated the roguelikes in the Rainy Day development branch (that is Rainy Day, SpelunkyRL and CvRL2) to use webpack + electron instead of the old web-only browserify build stack.

This means downloadable versions for all of them will be available soon, just need to add support to check for latest versions (a must for downloadable games IMO).

Additionally, this opens the door to add graphics to them; I’m initially thinking on adding them to CvRL2, since it’s probably the one that would benefit the most.