Create separate extension(s)

[DavidLeoni]

evoscratch-vm is an engine and evoscratch-gui is an application. It would be better if we could extract our code into extension project(s) as much as possible separate from them. Extracted project(s) could be then published to npm as modules that anybody can load as dependency into their custom scratch-gui and scratch-vm.

For now it is not strictly necessary, but maybe @raffaelepojer you can search around if you find some reference/example on how to do it