Add Swift Package Manager support

Package.swift

@@ -0,0 +1,44 @@
+// swift-tools-version: 5.8
+
+import PackageDescription
+
+let package = Package(
+    name: "spdlog",
+    platforms: [
+        .iOS(.v12),
+        .macOS(.v10_14),
+    ],
+    products: [
+        .library(
+            name: "spdlog",
+            targets: ["spdlog"]),
+    ],
+    dependencies: [
+    ],
+    targets: [
+        .target(
+            name: "spdlog",
+            dependencies: [],
+            path: ".",
+            exclude: [
+              "bench",
+              "cmake",
+              "example",
+              "logos",
+              "tests",
+              "scripts",
+            ],
+            sources: [
+              "src",
+              "include",
+            ],
+            publicHeadersPath: "include",
+            cxxSettings: [
+                .headerSearchPath("spdlog"),
+                .define("SPDLOG_COMPILED_LIB"),
+            ],
+            linkerSettings: []
+        ),
+    ],
+    cxxLanguageStandard: .cxx11
+)