[ecashaddrjs] Change comment references to bitcoincash: to ecash: in cashaddr.js

Summary:
T3039

Depends on D13475

Since this repo is focused primarily on ecash, make commentary references more appropriate.

Test Plan:
Confirm that changes are only to comments in cashaddr.js
npm test all pass

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13476