fix(prompt-editor): resolve initial cursor position and auto-newline … (#8511)

### What problem does this PR solve?

In web folder's prompt-editor component, when entering content for the
first time, the cursor position is abnormal and it will automatically
wrap

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

Co-authored-by: leonlai <owllai123456>

web/src/components/prompt-editor/variable-picker-plugin.tsx

@@ -224,6 +224,9 @@ export default function VariablePickerMenuPlugin({
       }
 
       $getRoot().clear().append(paragraph);
+      if ($isRangeSelection($getSelection())) {
+        $getRoot().selectEnd();
+      }
     },
     [findLabelByValue],
   );