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/unusedParam/native.kt

@@ -0,0 +1,18 @@
+native
+fun foo(a: String): Int = noImpl
+
+native
+fun Int.foo(a: String): Int = noImpl
+
+native
+class Bar(b: Int, c: Char) {
+    fun baz(d: Int) {}
+}
+
+native
+object Obj {
+    fun test1(e: String) {}
+    object Nested {
+        fun test2(g: Int) {}
+    }
+}