Added support of Swift Package Manager

This allows to add spdlog as dependency to other Swift Packages or Xcode projects

.gitignore

@@ -93,3 +93,4 @@ cmake-build-*/
 # macos
 *.DS_store
 *.xcodeproj/
+.swiftpm/xcode

Package.swift

@@ -0,0 +1,32 @@
+
+// swift-tools-version: 5.8
+
+import PackageDescription
+
+let package = Package(
+    name: "spdlog",
+    platforms: [
+        .macOS(.v11),
+        .iOS(.v13)
+    ],
+    products: [
+        .library(
+            name: "spdlog",
+            targets: ["spdlog"]),
+    ],
+    targets: [
+        .target(name: "spdlog",
+                path: "",
+                exclude:[
+                ],
+                sources:[
+                    "src"
+                ],
+                publicHeadersPath:"include",
+                cSettings: [
+                    .define("SPDLOG_COMPILED_LIB"),
+                ]
+               )
+    ],
+    cxxLanguageStandard: .cxx20
+)