[landing][lib][web] Add buffer polyfill for webworkers

Summary: The updated siwe library (which will be added in the next diff) now relies on a buffer polyfill. This diff adds the buffer polyfill to webworkers.

Test Plan: Confirmed that the siwe library no longer printed a "buffer not found" error when loading web locally

Reviewers: ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11557