Support new `splitViewId` in `tabs.Tab` (Firefox >= 149)

[ariasuni]

Feature Request

https://blog.mozilla.org/addons/2026/04/23/webextensions-api-changes-firefox-149-152/

Firefox 149 introduces a new read-only splitViewId property on the tabs.Tab object to expose Firefox’s new split view feature (where two tabs are displayed side-by-side in one window).

I’ve read quickly but I don’t think the other changes require a change in WXT.

Is your feature request related to a bug?

N/A

What are the alternatives?

N/A

Additional context

N/A