Add ExperimentalContracts opt-in

Author: igoriakovlev

src/wasmJsMain/kotlin/timeZoneWidget.kt

@@ -5,6 +5,7 @@ import kotlinx.dom.appendText
 import org.w3c.dom.Element
 import org.w3c.dom.HTMLInputElement
 
+@kotlin.contracts.ExperimentalContracts
 fun Element.addTimeZoneWidget() {
     appendElement("div") {
         appendText("Time in ")