[herald] New formatPrice function to prep prices for tg msgs

Summary:
T2972

Depends on D13641

Add new function to format price with correct fiat currency symbol, if available

Test Plan:
npm test
npm run getCoingeckoPrices and note prices appear with currency symbol

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13642