Multiview support

[wusyong]

At present, Servo is designed with a single top-level browsing context, typically corresponding to a web page as viewed by users. We aim to evolve this architecture to accommodate multiple top-level browsing contexts. By introducing this capability, a single window could host several browsing contexts simultaneously, effectively emulating a multi-tabbed browser interface. This enhancement would not only facilitate a multi-tabbed viewing experience but also potentially allow for the customization of the window’s user interface, incorporating additional browsing contexts that overlay or complement the standard web page display.

The following are the related issues, commits, and PRs:

• https://github.com/servo/servo/issues/30593
• https://github.com/servo/servo/pull/31417