Add eslint config file

[joemaller]

This is in use on the IOP site. Should probably just add it:

//.eslint.js
module.exports = {
  env: {
    es2021: true,
    node: true,
  },
  extends: ["eslint:recommended", "plugin:react/recommended"],
  parserOptions: {
    ecmaFeatures: { jsx: true },
    ecmaVersion: "latest",
    sourceType: "module",
  },
  plugins: ["react"],
  rules: {
    "react/react-in-jsx-scope": "off",
    "sort-imports": ["error", { allowSeparatedGroups: true }],
  },
};

[joemaller]

If we do this, we'll also need to include eslint and eslint-plugin-react in the default package.json file