include `module` field in package.json (#3763)
include `module` field in package.json (#3763)

makes it easy to just `import { Socket } from 'phoenix'` when using rollup.
michalmuskala/phoenix,nshafer/phoenix,nshafer/phoenix,jwarwick/phoenix,jwarwick/phoenix,rafbgarcia/phoenix,phoenixframework/phoenix,0x00evil/phoenix,0x00evil/phoenix,phoenixframework/phoenix,optikfluffel/phoenix,rafbgarcia/phoenix,eksperimental/phoenix,michalmuskala/phoenix,rafbgarcia/phoenix,phoenixframework/phoenix,optikfluffel/phoenix,eksperimental/phoenix,optikfluffel/phoenix,nshafer/phoenix,0x00evil/phoenix