[ecashaddrjs] Adding existing library to monorepo as-is

Summary:
T2848

This repo is used by exchanges working with ecash. Should be maintained and tracked in the monorepo.

Test Plan: Compare to existing master branch of codebase at https://github.com/BytesOfMan/ecashaddrjs, same except .git folder is removed

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13386