Use webpack --watch for live reload

[EverlastingBugstopper]

Instead of using notify to tell webpack to build a script from scratch, we should instead watch the output of webpack, and shell out to webpack --watch. Then, when a new build is complete, wrangler should reupload the Worker. This can already be achieved by an end user using the javascript type and watching with webpack --watch

[EverlastingBugstopper]

i'm pretty sure this already happens, we just weren't taking advantage of it. closed by #1269 (i think)