[Cashtab] Implement typescript

Summary:
Commence converting Cashtab to typescript by updating 2 simple files, appConfig and formatting. 2 files because formatting has functions, which are unit-tested, so we can confirm the implementation does not break jest --- and formatting imports appConfig. Seemed like the smallest possible change.

Update linters and webpack settings to support ts

Going forward, will be a big process to get Cashtab fully converted. But we can now use typescript for any new developments. It's particularly important for working with agora and chronik-client.

Test Plan: npm test, npm run build works

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17053