Ensure a copy of the externals is returned when generating a Webpack config.

Co-Authored-By: deAtog <[email protected]>

Author: Lyrkan

lib/config-generator.js

@@ -107,7 +107,7 @@ class ConfigGenerator {
             config.resolve.alias['react-dom'] = 'preact-compat';
         }
 
-        config.externals = this.webpackConfig.externals;
+        config.externals = [...this.webpackConfig.externals];
 
         return config;
     }