Factor out shared webpack config into lib/webpack/shared.cjs

Summary: We're going to use this in the upcoming landing project.

Test Plan: Make sure Webpack still builds and server still returns a working webpage on both dev and prod

Reviewers: atul, palys-swm

Reviewed By: atul

Subscribers: KatPo, zrebcu411, Adrian, subnub

Differential Revision: https://phabricator.ashoat.com/D963