fix(wc): don't overwrite user-defined externals (#2831)

Author: LinusBorg

packages/@vue/cli-service/lib/commands/build/resolveWcConfig.js

@@ -56,6 +56,7 @@ module.exports = (api, { target, entry, name }) => {
     // externalize Vue in case user imports it
     config
       .externals({
+        ...config.get('externals'),
         vue: 'Vue'
       })