[ecashaddrjs] Support decoding prefixless addresses if checksum is correct

Summary:
T3039

Depends on D13474

Test Plan: Review change to function and new unit test. npm test and all pass.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13475