[Wasm] Add JsAny.unsafeCast

Replacement for Dynamic.unsafeCast, but limited to JsAny types where
it makes sense.

libraries/stdlib/wasm/src/kotlin/js/JsAny.kt

@@ -5,7 +5,19 @@
 
 package kotlin.js
 
+import kotlin.wasm.internal.ExcludedFromCodegen
+import kotlin.wasm.internal.WasmNoOpCast
+import kotlin.wasm.internal.implementedAsIntrinsic
+
 /**
  * Any JavaScript value except null or undefined
  */
-public external interface JsAny
+public external interface JsAny
+
+/**
+ * Cast JsAny to other Js type without runtime check
+ */
+@WasmNoOpCast
+@ExcludedFromCodegen
+public fun <T : JsAny> JsAny.unsafeCast(): T =
+    implementedAsIntrinsic