[Cashtab] Move dev dependencies to dev dependencies

Summary:
Depends on D15247

Improve dependency organization by moving dev-related dependencies to the appropriate key.

Note: since Cashtab is built with webpack, all "used" dependencies get into the codebase no matter where they are stored. So this is mostly to improve dev readability.

Test Plan: confirm no change to package-lock.json

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15248