[ecashaddrjs] Implement typescript linting and add to arclint

Summary: Now that webpack is out, we can properly implement typescript linting. Do this, and add to arc lint.

Test Plan: npm test, arc lint -- modules/ecashaddrjs/**/*

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17183