Add warning for initializers with obsolete syntax

Expected 'init' keyword before class initializer

compiler/testData/diagnostics/tests/AnonymousInitializers.kt

@@ -1,18 +1,18 @@
 trait NoC {
-  <!ANONYMOUS_INITIALIZER_IN_TRAIT!>{
+  <!ANONYMOUS_INITIALIZER_IN_TRAIT!>init {
 
   }<!>
 
   val a : Int get() = 1
 
-  <!ANONYMOUS_INITIALIZER_IN_TRAIT!>{
+  <!ANONYMOUS_INITIALIZER_IN_TRAIT!>init {
 
   }<!>
 }
 
 class WithC() {
   val x : Int
-  {
+  init {
     $x = 1
     <!UNRESOLVED_REFERENCE!>$y<!> = 2
     val <!UNUSED_VARIABLE!>b<!> = x
@@ -21,7 +21,7 @@ class WithC() {
 
   val a : Int get() = 1
 
-  {
+  init {
     val <!UNUSED_VARIABLE!>z<!> = <!UNRESOLVED_REFERENCE!>b<!>
     val <!UNUSED_VARIABLE!>zz<!> = x
     val <!UNUSED_VARIABLE!>zzz<!> = <!NO_BACKING_FIELD_CUSTOM_ACCESSORS!>$a<!>