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/diagnostics/tests/extensions/classObject.kt

@@ -1,7 +1,7 @@
 trait Tr
 
-class A { class object }
-class B { class object : Tr }
+class A { default object }
+class B { default object : Tr }
 
 fun Any.f1() {}
 fun Any?.f2() {}