[JS] Make JsExport experimental

- JsExport was meant to be experimental from the beginning but
annotation was missing
- Add missing @SinceKotlin

libraries/stdlib/js/src/kotlin/annotations.kt

@@ -161,12 +161,27 @@ public annotation class JsNonModule
 @Target(AnnotationTarget.FILE)
 public annotation class JsQualifier(val value: String)
 
+/**
+ * Marks experimental JS export annotations.
+ *
+ * Note that behaviour of these annotations will likely be changed in the future.
+ *
+ * Usages of such annotations will be reported as warnings unless an explicit opt-in with
+ * the [UseExperimental] annotation, e.g. `@UseExperimental(ExperimentalJsExport::class)`,
+ * or with the `-Xuse-experimental=kotlin.js.ExperimentalJsExport` compiler option is given.
+ */
+@Experimental(level = Experimental.Level.WARNING)
+@SinceKotlin("1.3")
+public annotation class ExperimentalJsExport
+
 /**
  * Exports top-level declaration.
  *
  * Used in future IR-based backend.
  * Has no effect in current JS backend.
  */
+@ExperimentalJsExport
+@SinceKotlin("1.3")
 @Retention(AnnotationRetention.BINARY)
 @Target(CLASS, PROPERTY, FUNCTION, FILE)
 public annotation class JsExport