Polyfills for EventTarget and CustomEvent

Closes #756 (closed)

CustomEvent is needed for IE https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent#Browser_compatibility, EventTarget for Safari https://developer.mozilla.org/en-US/docs/Web/API/EventTarget#Browser_compatibility

src/lib/event_target_polyfill.js

+import EventTargetPolyfill from '@ungap/event-target'
+
+try {
+  /* eslint-disable no-new  */
+  new EventTarget()
+  /* eslint-enable no-new  */
+} catch (e) {
+  window.EventTarget = EventTargetPolyfill
+}