[Cashtab] Use typescript to build extension files instead of browserify

Summary:
The extension is built with a legacy bundler, browserify, to preserve a file structure required for legacy extension dev. We can do the same thing with typescript, and we should, as it makes everything more maintainable.

Implement.

Test Plan:
npm run extension builds, test extension locally

extension / browser interaction works as before, can test at https://components.cashtab.com/ (get address, open payments, etc)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18218