Espruino plugin for Rollup module bundler

Yeah, I know - sorry about that PR. I've been on the fence about adding the big chunk of extra code for far too long, but I'll try and get it in this/next week. It's just a shame we can't do some kind of 'load on demand' thing.

It should be possible to lazy-load the closure compiler. My main concern was the time it takes to execute and the lack of any visual hint on that the browser is working on something.. Perhaps wasm would help eventually. Also this could be done in a webworker so that it would not block the js event queue.