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/classes/classObjectInterface.kt

@@ -1,7 +1,7 @@
 class C() {
   fun getInstance(): Runnable = C
 
-  class object: Runnable {
+  default object: Runnable {
     override fun run(): Unit { }
   }
 }