[Cashtab] Remove babel settings from package.json

Summary:
Depends on D15257

Move babel settings to own file. It's otherwise not obvious they would be in package.json.

Test Plan:
Re-insert key and npm run build
Delete key and npm run build

The hashes are unchanged (can see in extension of build/static/js/*.js files)

npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15258