Add Window scene, openWindow action, dismissWindow action, fix window close handling, and add multi-window support to WindowGroup

[kaascevich]

This PR:

• Adds a Window scene and its associated WindowNode
  • Only one instance of Window can be open at a time
  • Window requires a title and an ID
• Implement multi-window support for WindowGroup
• Implements the dismissWindow environment action, which closes the enclosing window
• Implements the openWindow environment action, which opens a Window or WindowGroup given its ID
• Factors out the window setup and view graph updating code into a new (internal) WindowReference type
• Adds the defaultLaunchBehavior(_:) scene modifier, which controls whether scenes appear on app launch
  • The automatic option matches the current behavior, namely that WindowGroups are opened on app launch but Windows aren't
• Adds the supportsMultipleWindows environment value, which can be used to check whether the current platform supports multi-window
  • As far as the implementation is concerned, they all do, but practically speaking mobile backends don't typically support it