Move code actions to their own module

As we are collecting more syntactic code actions, I think it makes sense to organize them in their own module instead of storing them in their own directory inside `SwiftLanguageService`, if possible. As this is a learning-oriented task, please don’t address this purely by using an AI tool, as described in https://github.com/swiftlang/project-operations/blob/main/contributing/ai-tools.md#learning-oriented-contributions.