[Cashtab] Use chrome API instead of extensionizer

Summary:
"extensionizer" is a polyfill dep that allows a browser extension to work for both Chrome and Firefox APIs. However, we have never supported non-Chrome browsers for this extension, and extensionizer has not been updated for 7 years.

Stop using extensionizer and instead directly use the chrome API.

Doing this as a precursor to other extension improvements (use extension storage API instead of indexedDb, implement typescript, more dev-friendly app interactions, etc)

Test Plan: npm run extension to build locally, use it locally, it works; npm test to confirm nothing breaks

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D18214