[lib] Include COMM_ALCHEMY_KEY in web Webpack build as well

Summary:
In D6113 we add process.env.COMM_ALCHEMY_KEY to be accessible from landing. But for ENG-1969, we'll need the same key available in the web build. This diff moves the logic to lib/webpack/shared.cjs so all Webpack bundles have access.

Depends on D6113

Test Plan: Tested together with later diffs in the stack

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6114