Replace 'class object' with 'default object' in renderers and test data

Includes changes to decompiled text
Old syntax is used in builtins and project code for now

compiler/testData/asJava/lightClassStructure/ClassObject.kt

@@ -1,6 +1,6 @@
 package test
 
 class WithClassObject {
-  class object {
+  default object {
   }
 }

compiler/testData/asJava/lightClassStructure/PlatformStaticMethodsGenerics.kt

@@ -3,7 +3,7 @@ package test
 import kotlin.platform.platformStatic
 
 class PlatformStaticClass {
-    class object {
+    default object {
         platformStatic
         fun inClassObject<T>() {}
     }

compiler/testData/asJava/lightClasses/nullabilityAnnotations/ClassObjectField.kt

@@ -1,7 +1,7 @@
 // ClassObjectField
 
 class ClassObjectField {
-    class object {
+    default object {
         val x: String? = ""
         private val y: String? = ""
     }

compiler/testData/asJava/lightClasses/nullabilityAnnotations/TraitClassObjectField.kt

@@ -1,7 +1,7 @@
 // TraitClassObjectField
 
 trait TraitClassObjectField {
-    class object {
+    default object {
         val x: String? = ""
         private val y: String? = ""
     }