[ecash-opreturn] New npm library for parsing OP_RETURN in ecash app dev

Summary: These functions were developed for correctly parsing the latest alias spec. On completion, realized need basically the same thing to continue progress on ecash-herald. Should be a library.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13993