[Request]: Add formal support for Swift Package Manager (SPM)

### Plugin

connectivity_plus

### Use case

I am migrating my Flutter project to use the new Swift Package Manager (SPM) feature (flutter config --enable-swift-package-manager). However, when SPM is enabled, the build fails because the native iOS module is not found.

The error occurs in GeneratedPluginRegistrant.swift:
Module 'connectivity_plus' not found

I have confirmed that disabling SPM and reverting to CocoaPods fixes the issue, but I would like to use SPM as it is becoming the standard for Flutter iOS development.

### Proposal

Please update connectivity_plus to be fully compatible with Swift Package Manager. This involves:

Adding a valid Package.swift if missing.

Ensuring the Darwin (iOS/macOS) source files are structured to be recognized as a standalone module by Xcode when fetched via SPM.

Verifying that GeneratedPluginRegistrant can import the module correctly without CocoaPods header maps.