[JS IR] Fix referencing Kotin variables in inline JS code

Fixed by outlining JS code that uses Kotlin variables making usages of
these locals explicit and preventing bugs due to one-sided variable renaming.

This prevents using Kotlin variables as lvalue in JS code.

libraries/stdlib/js-ir/runtime/kotlinJsHacks.kt

@@ -12,3 +12,19 @@ internal fun <T : Enum<T>> enumValuesIntrinsic(): Array<T> =
 @PublishedApi
 internal fun <T : Enum<T>> enumValueOfIntrinsic(@Suppress("UNUSED_PARAMETER") name: String): T =
     throw IllegalStateException("Should be replaced by compiler")
+
+
+/**
+ * Implements annotated function in JavaScript.
+ * [code] string must contain JS expression that evaluates to JS function with signature that matches annotated kotlin function
+ *
+ * For example, a function that adds two Doubles:
+ *
+ *      @JsFun("(x, y) => x + y")
+ *      fun jsAdd(x: Double, y: Double): Double =
+ *          error("...")
+ *
+ * Code gets inserted as is without syntax verification.
+ */
+@Target(AnnotationTarget.FUNCTION, AnnotationTarget.PROPERTY_GETTER, AnnotationTarget.PROPERTY_SETTER)
+internal annotation class JsFun(val code: String)