Add 'Open in new tab' option to Editor split button dropdown

Follow-up from [#6670](https://github.com/FlowFuse/flowfuse/pull/6670) feedback.

Users might not know about middle-click/Ctrl+click to open the immersive editor in a new tab. While this is standard browser behavior, not all users are aware of it. Add an explicit **"Open in new tab"** option to the split button dropdown so it's discoverable for all users.

The dropdown in `EditorLink.vue` would become:
- **Open in new tab** — opens immersive editor in a new browser tab
- **Open Direct URL** — existing behavior

### Potential solution

Add a new entry to the `dropdownOptions` computed property in `frontend/src/pages/instance/components/EditorLink.vue` that calls `window.open()` with the immersive editor route (or the fallback standard editor URL for older Node-RED versions).