[herald] Move non-config constants to separate file

Summary:
T2972

Following feedback from D13875, creating a separate constants.js file to hold constant values that are not related to config.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13930