[Cashtab] New formatting method for getting decimals for very small xec values

Summary:
Agora partials can be priced in nanosatoshis. We want a way to render these as localeStrings with appropriate minimimumFractionDigits.

Add new formatting helper functions.

Not implemented in this diff, see D16846 for the purpose.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16925