refactor: use an object spread instead of Object.assign (#2395)

Author: noritaka1166

lib/command.js

@@ -245,11 +245,10 @@ class Command extends EventEmitter {
   configureOutput(configuration) {
     if (configuration === undefined) return this._outputConfiguration;
 
-    this._outputConfiguration = Object.assign(
-      {},
-      this._outputConfiguration,
-      configuration,
-    );
+    this._outputConfiguration = {
+      ...this._outputConfiguration,
+      ...configuration,
+    };
     return this;
   }