[ecash-lib] Add pushNumberOp to push minimally encoded numbers

Summary:
Add a companion function to pushBytesOp that encodes numbers. It handles signs and little-endian conversion by replicating the CScriptNum serialization algorithm.

Depends on D16529

Test Plan: npm run test

Reviewers: tobias_ruck, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: tobias_ruck, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16531