[Cashtab] [p1 remove BCH.getByteCount] Create an equivalent function with unit tests

Summary:
T2730

Create a new function getCashtabByteCount, which returns the exact same value as BCH.BitcoinCash.getByteCount()

Test Plan: Review new function, unit tests, and npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12376