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/testsWithJsStdLib/native/optionlBody/native.kt

@@ -23,7 +23,7 @@ trait T {
     fun foo()
     fun bar() {}
 
-    class object {
+    default object {
         val baz: Int
         val boo: Int = noImpl
 
@@ -40,7 +40,7 @@ class C {
     fun foo()
     fun bar() {}
 
-    class object {
+    default object {
         val baz: Int
         val boo: Int = noImpl