[ecash-herald] Format sent xec amounts

Summary:
T2972

Add a new function to convert satoshis to a formatted string appropriate for a msg.

Implement for normal XEC txs. Not implemented (yet) for encrypted cashtab msgs, airdrops, and fusions as these functions preferencially display fiat values if they have them. Will add here after adding a function to format fiat values.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14079