Change @babel/preset-env modules to 'auto'

[rossta]

@babel/preset-env already defaults to 'auto' and encourages its use when using babel with a bundler such as webpack.

According to babel docs (source: https://babeljs.io/docs/en/babel-preset-env#modules)

Setting this to false will preserve ES modules. Use this only if you intend to ship native ES Modules to browsers. If you are using a bundler with Babel, the default modules: "auto" is always preferred.

I believe this would also be consistent with create-react-app which appears to use the default.

Given this info, I believe Webpacker should follow suit.

[justin808]

@gauravtiwari @rossta If the default is 'auto', why specify it?

create-react-app specifies less:

https://github.com/facebook/create-react-app/blob/42dcf79b43171449a116ddcb51dce7b043cdb1d2/packages/babel-preset-react-app/create.js#L80-L88

        require('@babel/preset-env').default,
        {
          // Allow importing core-js in entrypoint and use browserlist to select polyfills
          useBuiltIns: 'entry',
          // Set the corejs version we are using to avoid warnings in console
          corejs: 3,
          // Exclude transforms that make all code slower
          exclude: ['transform-typeof-symbol'],
        },