fix(next): live preview popup triggers leave without saving modal (#12947)

Partially closes #12121.

When you edit a document in Live Preview using the default iframe
window, then attempt to open the window as a popup, the
`LeaveWithoutSaving` modal will appear.

This is because the `usePreventLeave` hook watches for anchor tags that
might cause a page navigation, and rightfully warns the user before they
navigate away and lose their changes. The reason the popup button
triggers this hook is because it uses an anchor tag with an href for
accessibility, which fires events that are caught and processed by the
hook.

The fix is to add the `target="_blank"` attribute here so that the hook
understands that these events do not navigate the user away from the
page and can be ignored.

---
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used, see below:
  - https://app.asana.com/0/0/1210643905956946

packages/next/src/views/LivePreview/Toolbar/Controls/index.tsx

@@ -114,6 +114,7 @@ export const ToolbarControls: React.FC<EditViewProps> = () => {
           e.preventDefault()
           setPreviewWindowType('popup')
         }}
+        target="_blank"
         type="button"
       >
         <LinkIcon />