[Cashtab] Wallet methods to ts

Summary:
Needed for ts component to manage NFT collections. Convert in limited diff here as part of broader effort to bring Cashtab to ts.

Now that we have stable types from ChronikClient, opportunity for more sophisticated and tested Wallet as a class, maybe a library for app devs. But this would be a separate effort.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D17080