Set selectionStart and selectionEnd correctly

Instead of defaulting to 0, we should put the selection always at the
end of the comment, else drag and dropping anything would insert it at
the very beginning

src/components/ExtendedTextarea.svelte

@@ -56,8 +56,8 @@
   /* eslint-enable prefer-const */
 
   let preview: boolean = $state(false);
-  let selectionStart = $state(0);
-  let selectionEnd = $state(0);
+  let selectionStart = $state(body.length);
+  let selectionEnd = $state(body.length);
   let draggingOver = $state(false);
   let dragEnterUnlistenFn: UnlistenFn | undefined = undefined;
   let dragLeaveUnlistenFn: UnlistenFn | undefined = undefined;