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/codegen/boxWithJava/classObjectAccessor/classObject.kt

@@ -1,12 +1,12 @@
 class A {
-    class object {
+    default object {
         val i1 = 1
         val i2 = 2
     }
 }
 
 class B {
-    class object Named {
+    default object Named {
         val i1 = 3
         val i2 = 4
     }