Fix to deep clone preset settings

Author: wooorm

lib/index.js

@@ -1124,8 +1124,6 @@ export class Processor extends CallableInstance {
   use(value, ...parameters) {
     const attachers = this.attachers
     const namespace = this.namespace
-    /** @type {Record<string, unknown> | undefined} */
-    let settings
 
     assertUnfrozen('use', this.frozen)
 
@@ -1143,11 +1141,6 @@ export class Processor extends CallableInstance {
       throw new TypeError('Expected usable value, not `' + value + '`')
     }
 
-    if (settings) {
-      // To do: structured clone?
-      namespace.settings = Object.assign(namespace.settings || {}, settings)
-    }
-
     return this
 
     /**
@@ -1184,8 +1177,10 @@ export class Processor extends CallableInstance {
       addList(result.plugins)
 
       if (result.settings) {
-        // To do: structured clone?
-        settings = Object.assign(settings || {}, result.settings)
+        namespace.settings = {
+          ...namespace.settings,
+          ...structuredClone(result.settings)
+        }
       }
     }