Add example of using spread operator in plugins

[jennifer-shehane]

From original issue: cypress-io/cypress#905

Show transform-object-rest-spread use in plugins api.

[rndmerle]

Just throwing this here since I struggled a bit to get it right:

yarn add -D @cypress/webpack-preprocessor @babel/preset-env @babel/plugin-proposal-object-rest-spread

const webpack = require('@cypress/webpack-preprocessor')
module.exports = webpack({
  webpackOptions: {
    module: {
      rules: [
        {
          test: /\.js$/,
          use: [{
            loader: 'babel-loader',
            options: {
              babelrc: false,
              configFile: false,
              presets: [['@babel/preset-env', { targets: { node: true } }]],
              plugins: ['@babel/plugin-proposal-object-rest-spread'],
            },
          }],
        },
      ],
    },
  },
})