[lib][landing][web] Move Webpack to lib

Summary:
The webpack NPM module is imported from lib only, so it makes sense for it to be there.

I considered leaving it in web and landing as well since I suspected perhaps webpack-dev-server or webpack-cli need it to be a peer dependency. But I didn't see any issues running the standard workflows (see test plan) so I figured it wasn't necessary.

Test Plan: Ran yarn dev and yarn prod in both web and landing. Ran the web app and landing page and played around

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6676