[web] Introduce copy-webpack-plugin webpack plugin

Summary:
LICENSE: MIT
GitHub (2.8k stars): https://github.com/webpack-contrib/copy-webpack-plugin

Context: https://linear.app/comm/issue/ENG-2831#comment-8c75b7cc, https://linear.app/comm/issue/ENG-2831#comment-1520f67c

We use copy-webpack-plugin in subsequent diff to make olm.wasm accessible to olm.js in the browser.

Test Plan: Tested implicitly by next diff

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6397