[herald] Rename chronik src file to websocket.js

Summary:
T3060

Stack to refactor ecash-herald so that all functions can be unit tested

Here, renaming chronik.js to websocket.js, which better reflects what is going on in this file.

Test Plan: Confirm no other imports are using chronik.js

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13614