Add eslint, using extendable config from `design-system`

[joshuagraber]

When design-system issue 40 is resolved:

• [ ] Add eslint. (npm i -D eslint)
• [ ] Create an eslintrc file and add @pdap-design-system/eslint-config to the extends property.
• [ ] Add lint and lint:fix scripts to package.json, following the model in data-sources