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

idea/testData/multiModuleQuickFix/nested/header/header.kt

@@ -0,0 +1,13 @@
+// "Create header class implementation for platform JVM" "true"
+
+header class <caret>WithNested {
+    fun foo(): Int
+
+    class Nested {
+        fun bar() = "Nested"
+    }
+
+    inner class Inner {
+        fun baz() = "Inner"
+    }
+}

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

@@ -0,0 +1,13 @@
+// "Create header class implementation for platform JVM" "true"
+
+header class WithNested {
+    fun foo(): Int
+
+    class Nested {
+        fun bar() = "Nested"
+    }
+
+    inner class Inner {
+        fun baz() = "Inner"
+    }
+}