add stop propagation and prevent default

src/event_ffi.mjs

···
 export function target(event) {
   return event.target;
 }
+
+export function preventDefault(event) {
+  return event.preventDefault;
+}
+
+export function stopPropagation(event) {
+  return event.stopPropagation;
+}

src/plinth/browser/event.gleam

···
 // event only knows about event target not Element/Document etc
 @external(javascript, "../../event_ffi.mjs", "target")
 pub fn target(event: Event) -> Dynamic
+
+@external(javascript, "../../event_ffi.mjs", "preventDefault")
+pub fn prevent_default(event: Event) -> Dynamic
+
+@external(javascript, "../../event_ffi.mjs", "stopPropagation")
+pub fn stop_propagation(event: Event) -> Dynamic