enableSingleRuntimeChunk() {
        webpackConfig.enableSingleRuntimeChunk();

        return this;
    }
Tell Webpack to output a separate runtime.js file.

This file must be included via a script tag before all
other JavaScript files output by Encore.

The runtime.js file is useful when you plan to include
multiple entry files on the same page (e.g. a layout.js entry
and a page-specific entry). If you are *not* including
multiple entries on the same page, you can safely disable
this - disableSingleRuntimeChunk() - and remove the extra script tags.

If you *do* include multiple entry files on the same page,
disabling the runtime.js file has two important consequences:
 A) Each entry file will contain the Webpack runtime, which
    means each contains some code that is duplicated in the other.
 B) If two entry files require the same module (e.g. jquery),
    they will receive *different* objects - not the *same* object.
    This can cause some confusion if you expect a "layout.js" entry
    to be able to "initialize" some jQuery plugins, because the
    jQuery required by the other entry will be a different instance,
    and so won't have the plugins initialized on it.

@returns {Encore}