Add tvOS support

[darkwater]

What problem does this solve or what need does it fill?

It's not currently possible to build Bevy for Apple's tvOS.

What solution would you like?

Add support for tvOS.

Because tvOS is very similar to iOS, most of the work will be changing cfg(target_os = "ios") into cfg(any(target_os = "ios", target_os = "tvos")), and most of it will be in wgpu and other dependencies.

What alternative(s) have you considered?

Don't support tvOS at all. It probably shouldn't be a priority target, but if the effort required is minimal, I think it would be worth it.

Additional context

I spent a night trying to get it to work, which was mostly patching dependencies to change cfg attributes, as well as configuring the Xcode project to build for tvOS instead of iOS. I got it to successfully build and install on my Apple TV 4K, but it would crash immediately upon launch. This was a late night about a week ago so I don't remember many details, but I intend to try again and document my findings here.

[james7132]

Marking this as blocked on gfx-rs/wgpu#5578.