Fix bug with dictation on iOS 18+

Dictation on iOS does not trigger composition events (https://bugs.webkit.org/show_bug.cgi?id=261764). Instead, it
triggers `beforeinput` events with `insertText`. During the dictation phase, it keeps the range anchored to the
cursor, and iOS may modify past text as new text adds context.

Once dictation is stopped, it starts sending `insertText` events with word fragments. When a past fragment is
altered, iOS sends a `beforeinput` event where `inputType` is null. In this case, perform a synchronous render
operation to set the editor to the correct state. Otherwise, scheduled render requests can process invalid
ranges and mess with the editor contents.

src/trix/controllers/level_2_input_controller.js

@@ -73,6 +73,11 @@ export default class Level2InputController extends InputController {
     beforeinput(event) {
       const handler = this.constructor.inputTypes[event.inputType]
 
+      // Handles bug with Siri dictation on iOS 18+.
+      if (!event.inputType) {
+        this.render()
+      }
+
       if (handler) {
         this.withEvent(event, handler)
         this.scheduleRender()