Quick-fix for header without implementation + a set of tests #KT-14908 Fixed

idea/testData/multiModuleQuickFix/sealed/header/header.kt

@@ -0,0 +1,7 @@
+// "Create header class implementation for platform JS" "true"
+
+header sealed class <caret>Sealed {
+    object Obj : Sealed()
+
+    class Klass(val x: Int) : Sealed()
+}

idea/testData/multiModuleQuickFix/sealed/header/header.kt.after

@@ -0,0 +1,7 @@
+// "Create header class implementation for platform JS" "true"
+
+header sealed class Sealed {
+    object Obj : Sealed()
+
+    class Klass(val x: Int) : Sealed()
+}