feat: add support for more package manager versions, use correct feature flag

Author: ph1ps

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 5.10
+// swift-tools-version: 5.9
 
 import PackageDescription
 
@@ -14,15 +14,15 @@ let package = Package(
   targets: [
     .target(
       name: "Deadline",
-      swiftSettings: [.enableUpcomingFeature("StrictConcurrency")]
+      swiftSettings: [.enableExperimentalFeature("StrictConcurrency")]
     ),
     .testTarget(
       name: "DeadlineTests",
       dependencies: [
         "Deadline",
         .product(name: "Clocks", package: "swift-clocks")
       ],
-      swiftSettings: [.enableUpcomingFeature("StrictConcurrency")]
+      swiftSettings: [.enableExperimentalFeature("StrictConcurrency")]
     ),
   ]
 )

Package@swift-5.8.swift

@@ -0,0 +1,26 @@
+// swift-tools-version: 5.8
+
+import PackageDescription
+
+let package = Package(
+  name: "swift-concurrency-deadline",
+  platforms: [.iOS(.v16), .macOS(.v13), .macCatalyst(.v16), .tvOS(.v16), .watchOS(.v9)],
+  products: [
+    .library(name: "Deadline", targets: ["Deadline"]),
+  ],
+  dependencies: [
+    .package(url: "https://github.com/pointfreeco/swift-clocks", from: "1.0.0")
+  ],
+  targets: [
+    .target(
+      name: "Deadline"
+    ),
+    .testTarget(
+      name: "DeadlineTests",
+      dependencies: [
+        "Deadline",
+        .product(name: "Clocks", package: "swift-clocks")
+      ]
+    ),
+  ]
+)

Package@swift-6.0.swift

@@ -0,0 +1,27 @@
+// swift-tools-version: 6.0
+
+import PackageDescription
+
+let package = Package(
+  name: "swift-concurrency-deadline",
+  platforms: [.iOS(.v16), .macOS(.v13), .macCatalyst(.v16), .tvOS(.v16), .watchOS(.v9), .visionOS(.v1)],
+  products: [
+    .library(name: "Deadline", targets: ["Deadline"]),
+  ],
+  dependencies: [
+    .package(url: "https://github.com/pointfreeco/swift-clocks", from: "1.0.0")
+  ],
+  targets: [
+    .target(
+      name: "Deadline",
+    ),
+    .testTarget(
+      name: "DeadlineTests",
+      dependencies: [
+        "Deadline",
+        .product(name: "Clocks", package: "swift-clocks")
+      ]
+    )
+  ],
+  swiftLanguageModes: [.v6]
+)

Sources/Deadline/Deadline.swift

@@ -1,4 +1,4 @@
-enum DeadlineState<T> {
+enum DeadlineState<T>: Sendable where T: Sendable {
   case result(Result<T, any Error>)
   case sleepWasCancelled
   case deadlineExceeded