[Cashtab] [xecjs-message upgrade p2/3] Sign msgs with ecash prefix in xecjs-message

Summary:
T2783

Depends on D12645

Sign msgs with ecash signature

Test Plan:
npm start
Sign a message
Confirm dev log displays 'Both methods not broken'
Verify a message
Confirm verification

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12646