chore: update Package.swift to Swift tools version 6.0

Package.swift

@@ -1,3 +1,32 @@
+// swift-tools-version: 6.0
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+    name: "damus",
+    platforms: [
+        .iOS(.v16),
+        .macOS(.v12)
+    ],
+    products: [
+        .library(
+            name: "damus",
+            targets: ["damus"]),
+    ],
     dependencies: [
-    .Package(url: "https://github.com/jb55/secp256k1.swift.git", branch: "main")
+        .package(url: "https://github.com/jb55/secp256k1.swift.git", branch: "main")
+    ],
+    targets: [
+        .target(
+            name: "damus",
+            dependencies: [
+                .product(name: "secp256k1", package: "secp256k1.swift")
+            ],
+            path: "damus"),
+        .testTarget(
+            name: "damusTests",
+            dependencies: ["damus"],
+            path: "damusTests"),
     ]
+)