[ecash-herald] Improve satsToFormattedValue fn

Summary:
We do not really want to round everything bigger than 1000 to no decimal places. Also, we should accept bigint for satoshis input.

Needed for D17180. Split out here for "one thing" isolation.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17181