[ecash-wallet] Move wallet class code and tests to wallet files, use an indext file for exports

Summary: We reasonably expect to have more code files that are separately tested. No need to cram the whole wallet into index.ts

Test Plan: npm test

Reviewers: #bitcoin_abc, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D17811