Add warning for initializers with obsolete syntax

Expected 'init' keyword before class initializer

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

@@ -1,7 +1,7 @@
 val y = 1
 
 class A() {
-    {
+    init {
         <!INACCESSIBLE_BACKING_FIELD!>$y<!> = 1
     }
 }