Add Swift Package Manager support

- Modified source files from and moved `Classes/*` to `window_manager/Sources/window_manager/*`
- Added `Package.swift`

packages/window_manager/macos/window_manager.podspec

@@ -13,7 +13,7 @@ A new flutter plugin project.
   s.license          = { :file => '../LICENSE' }
   s.author           = { 'LiJianying' => '[email protected]' }
   s.source           = { :path => '.' }
-  s.source_files     = 'Classes/**/*'
+  s.source_files     = 'window_manager/Sources/window_manager/**/*'
   s.dependency 'FlutterMacOS'
 
   s.platform = :osx, '10.11'

packages/window_manager/macos/window_manager/.build/workspace-state.json

@@ -0,0 +1,11 @@
+{
+  "object" : {
+    "artifacts" : [
+
+    ],
+    "dependencies" : [
+
+    ]
+  },
+  "version" : 6
+}

packages/window_manager/macos/window_manager/Package.swift

@@ -0,0 +1,33 @@
+// swift-tools-version:5.39
+import PackageDescription
+
+let package = Package(
+    name: "window_manager",
+    platforms: [
+        .macOS(.v10_11)
+    ],
+    products: [
+        .library(
+            name: "window_manager",
+            targets: ["window_manager"]
+        )
+    ],
+    dependencies: [],
+    targets: [
+        .target(
+            name: "window_manager",
+            dependencies: [],
+            resources: [
+                // If your plugin requires a privacy manifest, for example if it collects user
+                // data, update the PrivacyInfo.xcprivacy file to describe your plugin's
+                // privacy impact, and then uncomment these lines. For more information, see
+                // https://developer.apple.com/documentation/bundleresources/privacy_manifest_files
+                // .process("PrivacyInfo.xcprivacy"),
+
+                // If you have other resources that need to be bundled with your plugin, refer to
+                // the following instructions to add them:
+                // https://developer.apple.com/documentation/xcode/bundling-resources-with-a-swift-package
+            ]
+        )
+    ]
+)