Add script to build CashTab as browser extension

Summary: Build CashTab as Chrome or Brave browser extension

Test Plan:

1. npm run extension
2. Open Chrome or Brave
3. Navigate to chrome://extensions/ (or brave://extensions/)
4. Enable Developer Mode
5. Click "Load unpacked"
6. Select the extension/dist folder that was created with npm run extension

Reviewers: alcipir, deadalnix, #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8578