Watch a directory with webpack

I have webpack installed in my development environment to bundle my dependencies. In addition, I am using the webpack-merge-and-include-globally plugin to concatenate a bunch of js files and include in a script tag in the markup.

When I run webpack with the --watch flag, it watches my entry file (build/index.js) but I would also like it to watch all the files in the /build/ directory and any files or directories I place inside it - a "deep watch" of sorts.

How can I setup webpack to --watch an arbitrary directory, not associated with my entry file?

webpack.config.js

const path = require('path');
const TerserPlugin = require("terser-webpack-plugin");
const merge = require('webpack-merge-and-include-globally');

module.exports = {
    entry: "./build/index.js",
    module: {
        rules: [
            {
                test: /\.s[ac]ss$/i,
                use: [
                    "style-loader",
                    "css-loader",
                    "sass-loader",
                ],
            },
        ],
    },
    output: {
        path: path.resolve(__dirname,'dist'),
        filename: 'bundle.js'
    },

    plugins: [
        new merge({
            files: {
                "./app.js" : [
                    './build/js/app.js',
                    './build/js/controllers/*',
                ],
            },
        }),
    ],

    optimization: {
        minimize: process.env.NODE_ENV === 'production',
        minimizer: [new TerserPlugin()]
    },

    mode: process.env.NODE_ENV === 'production' ? 'production' : 'development'

}

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