[e.cash] Add header using default next lint settings

Summary:
T3182

Add header to code files in e.cash directory using next.js default settings

Note: Diffs planned after this to (1) fix known lint error about next/babel and to (2) expand lint dirs; right now, eslint is only linting nextjs default folders

Ref: https://nextjs.org/docs/pages/building-your-application/configuring/eslint

Test Plan:
Confirm the app still builds (lint errors will throw an error during build)

npm run lint and no errors

npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D14077