Add warning for initializers with obsolete syntax

Expected 'init' keyword before class initializer

compiler/testData/diagnostics/tests/backingField/ReadInAnonymous.kt

@@ -1,6 +1,6 @@
 class ReadByAnotherPropertyInitializer() {
     val a = 1
-    {
+    init {
         val <!UNUSED_VARIABLE!>x<!> = $a
     }
 }