[Cashtab] Generate targetOutputs for SLP genesis tx

Summary:
Add new function to generate a targetOutput for an SLP genesis tx.

This diff adds and unit tests the function. Function is not implemented.

Goal is to deprecate generateGenesisOpReturn and generateTokenTxOutput functions in cashMethods.js

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15060