[web] Add support for web workers to webpack

Summary:
Part of ENG-2826

This diff adds a new webpack configuration for generating web/service worker files. It should be easily extensible for future service workers (just add a new field in entry config). It also includes a basic service worker that logs when it installs and activates.

Test Plan: Run yarn dev and yarn prod in comm/web and check if the service worker file is correctly generated.

Reviewers: atul, tomek, kamil, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D6715