[e.cash] Add strict linting for next.js

Summary:
Pre-req for T3192

Follow documentation for adding lint to a next app per https://nextjs.org/docs/pages/building-your-application/configuring/eslint

Resolve raised lint errors

Test Plan: npm run lint and no lint errors. app builds and displays with no errors.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14069