Espruino plugin for Rollup module bundler

@Gordon There is https://github.com/espruino/EspruinoTools/pull/80 which I'd appreciate getting feedback on. Would that be a way to go to integrate or are other ideas, requirements or comments.

One known issue:

• It looks like the terser config is tool agressive and although I researched the options I didn't find a reasonable way for it to not to strip the onInit() method if not called (having the top_level mangling on at the same time for the best compression ratio).