Add warning for initializers with obsolete syntax

Expected 'init' keyword before class initializer

compiler/testData/diagnostics/tests/secondaryConstructors/expectedInitKeywordOnInitializer.kt

@@ -0,0 +1,10 @@
+class A {
+    val x: Int
+    val y: Int
+    <!INIT_KEYWORD_BEFORE_CLASS_INITIALIZER_EXPECTED!>{<!>
+    x = 1
+    }
+    init {
+        y = 1
+    }
+}

compiler/testData/diagnostics/tests/secondaryConstructors/expectedInitKeywordOnInitializer.txt

@@ -0,0 +1,10 @@
+package
+
+internal final class A {
+    public constructor A()
+    internal final val x: kotlin.Int
+    internal final val y: kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}