Introduce way to configure project-wide defaults for app config properties

[stackotter]

Some properties such as the Mac Catalyst interface idiom would be handy to be able to configure once at the top level of a Bundler.toml instead of once per app. This would be especially useful for projects with many apps such as SwiftCrossUI's example suite.

Prototype syntax

# ...

[[defaults.apps.overlays]]
condition = "platform(macCatalyst)"
interface_idiom = "mac" # Equivalent of Xcode's 'Optimize for Mac'

# ...