Fix webpack.config.js for v4/v5

[joemin]

Hey, thanks for a great tutorial! Super helpful for understanding what's given "for free" by create-react-app.

I think the webpack.config.js is still using the syntax for v3.

For the devServer key, I think it should be:

    devServer: {
        static : {
          directory : path.join(__dirname, "public/")
        },
        port: 3000,
        devMiddleware:{
           publicPath: "https://localhost:3000/dist/",
        },
        hot: "only",
    },

as shown in https://stackoverflow.com/questions/69102254/webpack-options-has-an-unknown-property-hotonly-invalid-options-object-dev-s

Thanks!