Add feature where users can choose the schemes to generate

Will be good a feature where the users could choose the schemes to be generated in the xcode project.

One solution is by command line argument. For example:
```
swift swiftplate/main.swift # create all schemes (iOS, macOS, watchOS, tvOS)
swift swiftplate/main.swift --ios # create schemes only for iOS
swift swiftplate/main.swift --ios --tvos # create schemes only for iOS and tvOS
```

Start at swiftplate/main.swift and trace how the Xcode project and its schemes are generated. Review the command-line argument handling and identify where the iOS, macOS, watchOS, and tvOS choices are applied. Done means the default still generates all schemes, while each documented flag limits generation to the requested platforms.

Written by the indexing model from the issue text.