[Cashtab] Better function to convert user-entered fiat prices to satoshis

Summary:
Add and test (but do not implement) a function to convert user-entered fiat amounts to satoshis of XEC

Ref D15662 to see where this is going. Some refactor work is required to consolidate Cashtab's send xec amount validation functions.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15670