Support npm pkgs in extension-specific files

Summary:
As more complicated features are added to the extension, npm packages will be required.
This diff compiles background.js and contentscript.js to accept npm dependencies.

Future features require this foundational dev infra change.

Test Plan:
npm i -g browserify
npm run extension and observe no errors in build
Navigate to chrome://extensions and update Cashtab or load web/cashtab/extension/dist as unpacked extension
Navigate to https://laughing-villani-8cfcaf.netlify.app/?path=/story/cashtabbutton--default and observe that BCHA buttons open prepopulated Cashtab window

There should be no functionality change in the extension from before.

Reviewers: majcosta, alcipir, #bitcoin_abc

Reviewed By: alcipir, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D8761