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/diagnostics/tests/inner/nestedClassAccessedViaInstanceReference.kt

@@ -1,24 +1,24 @@
 trait N { fun foo() = 1 }
 
 class WithClassObject {
-    class object {}
+    default object {}
 
     class Nested()
-    class NestedWithClassObject { class object : N }
+    class NestedWithClassObject { default object : N }
     enum class NestedEnum { A }
     object NestedObj : N { fun invoke() = 1 }
 }
 
 class WithoutClassObject {
     class Nested()
-    class NestedWithClassObject { class object : N }
+    class NestedWithClassObject { default object : N }
     enum class NestedEnum { A }
     object NestedObj : N { fun invoke() = 1 }
 }
 
 object Obj {
     class Nested()
-    class NestedWithClassObject { class object : N }
+    class NestedWithClassObject { default object : N }
     enum class NestedEnum { A }
     object NestedObj : N { fun invoke() = 1 }
 }