addExternals(externals) {
        webpackConfig.addExternals(externals);

        return this;
    }
Allow you to exclude some dependencies from the output bundles.

See https://webpack.js.org/configuration/externals/

For example:

```
Encore.addExternals({
    jquery: 'jQuery',
    react: 'react'
});
```

Or:

```
const nodeExternals = require('webpack-node-externals');

Encore.addExternals(
    nodeExternals()
);

// or add multiple things at once
Encore.addExternals([
    nodeExternals(),
    /^(jquery|\$)$/i
]);
```

@param {webpack.Externals} externals
@returns {Encore}