JS frontend: allow to omit body for native functions and initializer for native properties. Additionally suppress UNUSED_PARAM for new nativeX annotations too.

compiler/testData/diagnostics/testsWithJsStdLib/native/optionlBody/nativeSetter.kt

@@ -0,0 +1,13 @@
+nativeSetter
+fun String.foo(n: Int, v: Any)
+nativeSetter
+fun String.bar(n: Int, v: Any) {}
+
+
+native
+class C {
+    nativeSetter
+    fun foo(d: Double, v: Any): String?
+    nativeSetter
+    fun bar(d: Double, v: Any): String? = noImpl
+}