[ecashaddrjs] Support ecregtest prefix

Summary:
T3106

Add ecregtest as a valid prefix. Do not add it to VALID_PREFIXES_MAINNET

Test Plan: I did not add any specific unit tests for this prefix (similar to bchreg prefix). However, npm test to confirm existing functionality is not impacted.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13666