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/writeFlags/function/classObjectPrivate/privateFun.kt

@@ -2,7 +2,7 @@ class Foo {
 
   {Foo.test()}
 
-  class object {
+  default object {
     private fun test() {
 
     }

compiler/testData/writeFlags/function/classObjectPrivate/privateVal.kt

@@ -2,7 +2,7 @@ class Foo {
 
   {Foo.test}
 
-  class object {
+  default object {
     private val test = "String"
   }
 }

compiler/testData/writeFlags/function/classObjectPrivate/privateVar.kt

@@ -2,7 +2,7 @@ class Foo {
 
   {Foo.test}
 
-  class object {
+  default object {
     private var test = "String"
   }
 }