[ecash-lib] Add parseEmppScript method

Summary:
We have a number of methods for parsing individual pushes from an EMPP OP_RETURN, but we are missing a method to take the EMPP script and get the individual pushes.

Add it here.

Test Plan: npm test

Reviewers: #bitcoin_abc, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Subscribers: tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D18057