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/j+k/InnerClassFromJava.kt

@@ -6,7 +6,7 @@ public interface I extends Boo {
 
 // FILE: Baz.kt
 public trait Foo {
-    class object {
+    default object {
         public val EMPTY: Foo = object : Foo{}
     }
 }