Add a tool to migrate an xcodeproj to Skip

People keep asking about this, and the documentation around this https://skip.dev/docs/project-types/#existing_development basically says "start from scratch, copying over code into a fresh project created with skip init or skip create."

But I can dimly imagine a tool that could at least take a standard Xcode-generated SwiftUI xcodeproj (with no xcworkspace), convert it into an xcodeproj that depends on a SwiftPM module, moving the xcodeproj into a Darwin folder, and generate an Android directory for it.

It could perhaps run in phases. Perhaps the first phase would just SwiftPM-ify the whole thing, and then stop and let the user verify that their iOS app still works as expected. Then, maybe a second command could skip-ify one SwiftPM module, ensuring that it builds on Android.

(Some day, as part of an integration test, we could search Github for open-source pure SwiftUI apps and try to automatically migrate as many of them as we can.)

This would be a lot of work, but I think it could really open doors for Skip.

Begin with the existing-development section at skip.dev/docs/project-types/#existing_development and the skip init/skip create entry points. Define the migration phases and verify completion by preserving the iOS app while producing a SwiftPM-based project, Darwin folder, and Android directory; the issue does not identify implementation files or tests.