Export WorkboxEventMap from workbox-window

[jeffposnick]

R: @tropicadri
CC: @ivandotv

Fixes #2860

Exporting the existing event mapping seemed cleaner than exporting a bunch of events directly alongside the other symbols.

This should allow for use cases like:

import {Workbox, WorkboxEventMap} from 'workbox-window';

const handleWaiting = (event: WorkboxEventMap['waiting']) => {
  // ...
};

const wb = new Workbox('sw.js');
wb.addEventListener('waiting', handleWaiting);