documentation: Add example of passing options to a custom babel-preset

[johlug]

It took me a while to find out how to create a custom babel-preset, where I can pass options to it. It would be good if you can add an example to the documentation, in a way like this:

// babel-preset-myPreset on npm

module.exports = (api, opts) => {
  return {
    plugins: [
      "@babel/plugin-proposal-nullish-coalescing-operator",
      "@babel/plugin-proposal-optional-chaining"
    ],
    presets: [
      [
        "@babel/preset-env",
        {
          useBuiltIns: "entry",
          ...(opts && opts)
        }
      ]
    ]
  }
}

call in webpack configuration

// webpack configuratoin
{
  loader: 'babel-loader',
  options: {
    presets: [
      [
        'babel-preset-myPreset', {
          targets: {
            esmodules: true
          }
        }
      ]
    ]
  }
}