[ecashaddrjs] Add missing copyright notice to new file

Summary:
Overlooked in the ts implementation

ecashaddrjs at the moment does not easily lend itself to handling this with a plugin-set header, as different files have different authorship.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16746