[mock-chronik-client] Add module to monorepo

Summary:
As per feedback from D14418, many eCash apps are using an identical version of this mock chronik client, often tailored to specific needs.
This module is a fully fleshed out mock across the chronik-client APIs.

Separate diffs will be made to update existing apps in the monorepo to use this mock implementation.

Test Plan:
npm test
plonk README into https://jbt.github.io/markdown-editor/

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D14423