Date picker

[bbrk24]

Fixes #233

Each backend has its own shortcomings:

• UIKitBackend has weird availability constraints on each picker style, doesn't support hourMinuteAndSecond, and doesn't respect font color
  • Furthermore, UIDatePicker doesn't exist on tvOS
• AppKitBackend doesn't support the wheel picker style
• WinUIBackend doesn't support hourMinuteAndSecond, doesn't respect font color, and doesn't respect environment.timeZone (instead always using the current time zone)
• GtkBackend only supports the graphical picker style, doesn't respect environment.timeZone (instead always using UTC), and only supports picking the date, not the time
• Gtk3Backend is unimplemented as I have no way to test it.

[bbrk24]

The thought just occurred to me to observe NSSystemTimeZoneDidChange to properly redraw views that use @Environment(\.timeZone), but I'm unsure how that would work, and regardless I don't see an equivalent for the calendar.

[stackotter]

There’s a backend method called setEnvironmentChangeHandler or something like that. That’s where we currently observe system theme changes and would be where timezone observation should live as well.

[bbrk24]

Right, I'll go see if I can get that to work in UIKitBackend and AppKitBackend first, and then maybe the other backends. I don't think it'll necessarily work on Linux though

[bbrk24]

Marking this as draft because UIDatePicker (at least on iOS 17) completely misbehaves if the user changes the time zone in settings