[Cashtab] Deprecate legacy alias bytecount function

Summary:
Deprecate alias bytecount function for one in proper place with proper tests

The main goal here is to deprecate the legacy generateOpReturnScript function, still used here

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15058