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/cast/WhenWithExpression.kt

@@ -1,5 +1,5 @@
 class List<out T>(val size : Int) {
-    class object {
+    default object {
         val Nil = List<Nothing>(0)
     }
 }

compiler/testData/diagnostics/tests/cast/WhenWithExpression.txt

@@ -9,7 +9,7 @@ internal final class List</*0*/ out T> {
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 
-    internal class object Default {
+    internal default object Default {
         private constructor Default()
         internal final val Nil: List<kotlin.Nothing>
         public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean