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/nativeGetter.kt

@@ -0,0 +1,13 @@
+nativeGetter
+fun String.foo(n: Int): Int?
+nativeGetter
+fun String.bar(n: Int): Int? = noImpl
+
+
+native
+trait T {
+    nativeGetter
+    fun foo(d: Double): String?
+    nativeGetter
+    fun bar(d: Double): String? = noImpl
+}