Add Package.swift.

This makes it really easy to add SPIRV-Cross to an Xcode project.

Package.swift

@@ -0,0 +1,72 @@
+// swift-tools-version:5.5
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+// Copyright 2016-2021 The Khronos Group Inc.
+// SPDX-License-Identifier: Apache-2.0
+
+import PackageDescription
+
+let package = Package(
+    name: "SPIRV-Cross",
+    products: [
+        // Products define the executables and libraries a package produces, and make them visible to other packages.
+        .library(
+            name: "SPIRV-Cross",
+            targets: ["SPIRV-Cross"]),
+    ],
+    dependencies: [
+        // Dependencies declare other packages that this package depends on.
+        // .package(url: /* package url */, from: "1.0.0"),
+    ],
+    targets: [
+        // Targets are the basic building blocks of a package. A target can define a module or a test suite.
+        // Targets can depend on other targets in this package, and on products in packages this package depends on.
+        .target(
+            name: "SPIRV-Cross",
+            dependencies: [],
+            path: ".",
+            exclude: ["CMakeLists.txt",
+                      "CODE_OF_CONDUCT.adoc",
+                      "LICENSE",
+                      "LICENSES",
+                      "Makefile",
+                      "README.md",
+                      "appveyor.yml",
+                      "build_glslang_spirv_tools.sh",
+                      "checkout_glslang_spirv_tools.sh",
+                      "cmake",
+                      "format_all.sh",
+                      "gn",
+                      "main.cpp",
+                      "pkg-config",
+                      "reference",
+                      "samples",
+                      "shaders",
+                      "shaders-hlsl",
+                      "shaders-hlsl-no-opt",
+                      "shaders-msl",
+                      "shaders-msl-no-opt",
+                      "shaders-no-opt",
+                      "shaders-other",
+                      "shaders-reflection",
+                      "shaders-ue4",
+                      "shaders-ue4-no-opt",
+                      "test_shaders.py",
+                      "test_shaders.sh",
+                      "tests-other",
+                      "update_test_shaders.sh"],
+            sources: ["spirv_cfg.cpp",
+                      "spirv_cpp.cpp",
+                      "spirv_cross.cpp",
+                      "spirv_cross_c.cpp",
+                      "spirv_cross_parsed_ir.cpp",
+                      "spirv_cross_util.cpp",
+                      "spirv_glsl.cpp",
+                      "spirv_hlsl.cpp",
+                      "spirv_msl.cpp",
+                      "spirv_parser.cpp",
+                      "spirv_reflect.cpp"],
+            publicHeadersPath: "."),
+    ],
+    cxxLanguageStandard: .cxx14
+)