Add positive lookbehind to newPath regex

As pointed out by Slava in #2, the regular expression to replace ".next.js" with ".now.js" shouldn't be so tightly coupled to `Plugin.registerSourceHandler()`. This looks for a leading "." to make sure that we aren't replacing files like "next.js" or "filenext.js" (although previously we were trusting `Plugin.registerSourceHandler()` to take care of this.