What problem does this feature solve?

Webpack supports passing a function to output.filename, to be able to dynamically determine the output filename of an entry point.

In our case we are including some entries automagically using the builtins.html so we would like to include [contenthash] in the file name, but other bundles are being consumed by external scripts, so we would like to keep the output name static.

What does the proposed API of configuration look like?

From https://webpack.js.org/configuration/output/#outputfilename

module.exports = {
  //...
  output: {
    filename: (pathData) => {
      return pathData.chunk.name === 'main' ? '[name].js' : '[name]/[name].js';
    },
  },
};