JS backend: added delegate property support.

js/js.translator/testFiles/delegateProperty/cases/setAsExtensionFun.kt

@@ -0,0 +1,19 @@
+package foo
+class Delegate {
+    var inner = 1
+    fun get(t: Any?, p: PropertyMetadata): Int = inner
+}
+
+fun Delegate.set(t: Any?, p: PropertyMetadata, i: Int) { inner = i }
+
+class A {
+    var prop: Int by Delegate()
+}
+
+fun box(): String {
+  val c = A()
+  if(c.prop != 1) return "fail get"
+  c.prop = 2
+  if (c.prop != 2) return "fail set"
+  return "OK"
+}