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/compileKotlinAgainstKotlin/ClassObjectInEnum.A.kt

@@ -3,7 +3,7 @@ package library
 public enum class EnumClass {
     ENTRY
 
-    public class object {
+    public default object {
         public fun entry(): EnumClass = ENTRY
     }
 }

compiler/testData/compileKotlinAgainstKotlin/ClassObjectMember.A.kt

@@ -1,5 +1,5 @@
 class A {
-    class object {
+    default object {
         fun foo() = 42
         val bar = "OK"
     }