Update to webpack 5, remove gulp

Co-Authored-By: Mark McDowall <markus101@users.noreply.github.com>

frontend/build/webpack/css-variables-loader.js

@@ -0,0 +1,11 @@
+const loaderUtils = require('loader-utils');
+
+module.exports = function cssVariablesLoader(source) {
+  const options = loaderUtils.getOptions(this);
+
+  options.cssVarsFiles.forEach((cssVarsFile) => {
+    this.addDependency(cssVarsFile);
+  });
+
+  return source;
+};