Open a <model> as an immersive environment

Safari developed an experimental feature to open a model provided by the website as an immersive backdrop (https://github.com/WebKit/explainers/tree/main/spatial-backdrop). The way this currently works is through a <link> tag.

But since this experience shares many of the same properties as the <model> element, it may make sense for the <model> element to be augmented with a method to launch itself as a spatial backdrop / immersive environment (similarly to the fullscreen API). This would allow more control on the developer's side, and would also allow for an inline representation of the environment on platforms where <model> is supported but the immersive API is not.

Start by reading the spatial-backdrop explainer linked in the issue and compare its link-based behavior with the fullscreen API and the existing model element proposal. Define the intended method for launching a model as a spatial backdrop, including how inline representation should work when immersive APIs are unavailable. Done means the API scope and platform behavior are agreed in the model-element specification.