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

idea/testData/checker/ClassObjects.kt

@@ -1,10 +1,10 @@
 package Jet86
 
 class A {
-  class object {
+  default object {
     val x = 1
   }
-  <error descr="[MANY_DEFAULT_OBJECTS] Only one default object is allowed per class">class object Another</error> { // error
+  default <error descr="[MANY_DEFAULT_OBJECTS] Only one default object is allowed per class">object Another</error> { // error
     val x = 1
   }
 }
@@ -14,7 +14,7 @@ class B() {
 }
 
 object b {
-  <error descr="[DEFAULT_OBJECT_NOT_ALLOWED] A default object is not allowed here">class object</error> {
+  default <error descr="[DEFAULT_OBJECT_NOT_ALLOWED] A default object is not allowed here">object</error> {
     val x = 1
   }
   // error