[ecashaddrjs] Add function to support conversion to legacy format

Summary:
T3096

Going forward, we will use utxo-lib and bitcoinjs-message for javascript applications that create transactions and sign / verify msgs.

These libraries are robust and well maintained. However, they require address input format to be legacy.

Include a toLegacy function in ecashaddrjs to support future implementation of above libraries.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13656