[lib] Update clean-webpack-plugin to 4.0

Summary:

1. I moved this to lib... it's used in lib/webpack/shared.cjs, so I think that's where it should be
2. I updated from 3.0 to 4.0

Depends on D6677

Test Plan:

1. The only breaking changes listed in the release notes are that support for Node 8 and Webpack 4 was dropped
2. 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/D6678