Enable SwiftConcurrency feature in new compilers (#11)

# Enable SwiftConcurrency feature in new compilers

## ♻️ Current situation & Problem
This PR makes sure to enabled the `SwiftConcurrency` feature also with
Swift 6 compilers.
Further, we add SwiftLint if requested from the environment.

## ⚙️ Release Notes 
* Support SwiftConcurrency in Swift 6 compilers´
* Add SwiftLint build tool if `SPEZI_DEVELOPMENT_SWIFTLINT` environment
variable is set.

## 📝 Code of Conduct & Contributing Guidelines 

By submitting creating this pull request, you agree to follow our [Code
of
Conduct](https://github.com/StanfordSpezi/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/StanfordSpezi/.github/blob/main/CONTRIBUTING.md):
- [x] I agree to follow the [Code of
Conduct](https://github.com/StanfordSpezi/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/StanfordSpezi/.github/blob/main/CONTRIBUTING.md).

Package.swift

@@ -8,9 +8,17 @@
 // SPDX-License-Identifier: MIT
 //
 
+import class Foundation.ProcessInfo
 import PackageDescription
 
 
+#if swift(<6)
+let swiftConcurrency: SwiftSetting = .enableExperimentalFeature("SwiftConcurrency")
+#else
+let swiftConcurrency: SwiftSetting = .enableUpcomingFeature("SwiftConcurrency")
+#endif
+
+
 let package = Package(
     name: "SpeziFoundation",
     defaultLocalization: "en",
@@ -24,21 +32,45 @@ let package = Package(
     products: [
         .library(name: "SpeziFoundation", targets: ["SpeziFoundation"])
     ],
+    dependencies: swiftLintPackage(),
     targets: [
         .target(
             name: "SpeziFoundation",
             resources: [
                 .process("Resources")
             ],
             swiftSettings: [
-                .enableExperimentalFeature("StrictConcurrency")
-            ]
+                swiftConcurrency
+            ],
+            plugins: [] + swiftLintPlugin()
         ),
         .testTarget(
             name: "SpeziFoundationTests",
             dependencies: [
                 .target(name: "SpeziFoundation")
-            ]
+            ],
+            swiftSettings: [
+                swiftConcurrency
+            ],
+            plugins: [] + swiftLintPlugin()
         )
     ]
 )
+
+
+func swiftLintPlugin() -> [Target.PluginUsage] {
+    // Fully quit Xcode and open again with `open --env SPEZI_DEVELOPMENT_SWIFTLINT /Applications/Xcode.app`
+    if ProcessInfo.processInfo.environment["SPEZI_DEVELOPMENT_SWIFTLINT"] != nil {
+        [.plugin(name: "SwiftLintBuildToolPlugin", package: "SwiftLint")]
+    } else {
+        []
+    }
+}
+
+func swiftLintPackage() -> [PackageDescription.Package.Dependency] {
+    if ProcessInfo.processInfo.environment["SPEZI_DEVELOPMENT_SWIFTLINT"] != nil {
+        [.package(url: "https://github.com/realm/SwiftLint.git", .upToNextMinor(from: "0.55.1"))]
+    } else {
+        []
+    }
+}