[eCash webdev libraries] bitcoincashjs-lib as starting point for eCash stack

Summary:
T2782

This diff clones https://github.com/Permissionless-Software-Foundation/bitcoincashjs-lib

This is the current version of bitcoincashjs-lib as used by Cashtab.

Future diffs will customize and rename this library to focus on eCash.

Test Plan: Verify that no other folders are touched other than bitcoincashjs-lib

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12769