Clear highlights when changing the text.

I'm not a big fan of this behavior but we need some form of clearing and this is the simplest right now.

src/Editor.tsx

@@ -24,6 +24,7 @@ In the AST on the right, you can:
 function Editor({ defaultValue = default_org_source }) {
   function handleChange(event: React.ChangeEvent<HTMLTextAreaElement>) {
     setValue(event.target.value);
+    clearHighlights();
   }
 
   const [value, setValue] = useState(defaultValue);