Add support for visionOS

### What is the problem or limitation you are having?

Now that the Python support libraries support visionOS, Briefcase should also support that platform.

### Describe the solution you'd like

Add a platform backend to support tvOS. The implementation of this command will be almost identical to iOS, with some template and support apckage URL changes.

### Describe alternatives you've considered

-

### Additional context

The visionOS testbed project may also be useful as a point of reference.

Compare the existing iOS platform backend with the visionOS testbed project, focusing on the platform templates and support package URLs mentioned in the issue. The work is done when Briefcase can use a dedicated visionOS backend to produce a functioning visionOS application with the correct templates and support packages.

Written by the indexing model from the issue text.