[Cashtab] Introduce nanosat to xec conversion functions

Summary:
Because agora partial offers support pricing in nanosat per token sat, we need these functions to support user input in XEC (or fiat).

Write functions and unit tests. Functions are not implemented.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D16926