Add warning for initializers with obsolete syntax

Expected 'init' keyword before class initializer

compiler/testData/diagnostics/tests/regressions/kt1639-JFrame.kt

@@ -3,7 +3,7 @@ package test
 import javax.swing.JFrame
 
 class KFrame() : JFrame() {
-    {
+    init {
         val <!UNUSED_VARIABLE!>x<!> = this.rootPaneCheckingEnabled // make sure field is visible
     }
 }