Support ExtensionKit

[kabiroberai]

Now that #97 has landed, it's possible to create App Extensions, but only Foundation-based ones. We should support ExtensionKit-based extensions, the critical step being that we need to copy the extension into Foo.app/Extensions instead of Foo.app/PlugIns.

See #97 (comment)

[kabiroberai]

Might also be worth a quick spike to see if it's possible to declare ExtensionKit extension points. This is relevant for use cases like Enhanced Security Extensions. Not sure the effort for that would be worth it though. Possible that EX_ENABLE_EXTENSION_POINT_GENERATION relies on a custom tool that ships with Xcode, and also it seems like it relies on proprietary macros so this might require reimplementation a la #101 [uses result builders, not macros].

iOS 26b1-2 even enabled any app to declare an extension point for any other app, but this (unfortunately) seems to have been removed in b3. This is still officially supported on macOS, but xtool doesn't support building macOS apps atm anyway.