Allow multiplie targets

[westtrade]

Webpack can't use multiplie targets in one config https://webpack.js.org/concepts/targets/#multiple-targets, i think this patch can resolve this problem.

Example configuration

module.exports = {
    webpack(serverConfig, options) {
        const clientConfig = {
            target: 'web', // <=== can be omitted as default is 'web'
            entry: {
                client: ['src/client/index.js']
            },
            output: {
                path: 'build',
                filename: '[name].js',
                sourceMapFilename: '[name].map',
                publicPath: '/',
                libraryTarget: 'commonjs'
            }
        };

        return [serverConfig, clientConfig];
    }
};

[jaredpalmer]

this is a pretty cool idea

[Coriou]

Before finding this PR, I tried to do exactly this. My use case is I need a slightly special build for a cli "bin" and would have loved this to be supported

Any chance of seeing this one day ?