JS backend: basic support for class literals.

Added:
* the ability to get KClass using class literals (`::class`);
* the ability to get KClass from JsClass and vice versa;
* the ability to get simpleName.

 #KT-13345 Fixed

js/js.translator/testData/box/reflection/jsClass.kt

@@ -1,21 +1,12 @@
 package foo
 
-fun check(x: JsClass<*>, y: JsClass<*>, shouldBeEquals: Boolean = true) {
-    assertNotEquals(null, x)
-    assertNotEquals(null, y)
-    if (shouldBeEquals)
-        assertEquals(x, y)
-    else
-        assertNotEquals(x, y)
-}
-
 fun box(): String {
     check(jsClass<A>(), A().jsClass)
     check(jsClass<B>(), B().jsClass)
     check(jsClass<O>(), O.jsClass)
     assertNotEquals(null, jsClass<I>())
     check(jsClass<E>(), E.X.jsClass)
-    check(jsClass<E>(), E.Y.jsClass, shouldBeEquals = false)
+    check(jsClass<E>(), E.Y.jsClass, shouldBeEqual = false)
 // TODO uncomment after KT-13338 is fixed
 //    check(jsClass<E>(), E.Z.jsClass)