[Cashtab] New validation function for user input NFT list price

Summary:
Splitting this out from D16328 to simplify review

Add new (and, here, unimplemented) function to validate user input NFT list price. This function is designed to work with Cashtab input field, i.e. if there is an error, we want to return the error so the user can know what is wrong.

See D16328 for implementation context.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D16495