Add SwiftPM support (#1339)

Summary:
Using yoga in Swift

```swift
import UIKit
import yoga

let YGUndefined = Float.nan

UIGraphicsBeginImageContextWithOptions(CGSize(width: 1, height: 1), true, 0)
let scale = UIGraphicsGetCurrentContext()?.ctm.a ?? 1
UIGraphicsEndImageContext()

let config = YGConfigNew()
YGConfigSetUseWebDefaults(config, true)
YGConfigSetPointScaleFactor(config, Float(scale))

let root = YGNodeNewWithConfig(config)
YGNodeStyleSetFlexDirection(root, YGFlexDirectionRow)
YGNodeStyleSetFlexWrap(root, YGWrapWrap)
YGNodeStyleSetWidth(root, 200)

for i in 0..<4 {
    let leaf = YGNodeNewWithConfig(config)
    YGNodeStyleSetAspectRatio(leaf, 1)
    YGNodeStyleSetWidthPercent(leaf, 50)
    YGNodeInsertChild(root, leaf, UInt32(i))
}

YGNodeCalculateLayout(root, YGUndefined, YGUndefined, YGNodeStyleGetDirection(root))

for i in 0..<4 {
    let leaf = YGNodeGetChild(root, UInt32(i))
    let left = YGNodeLayoutGetLeft(leaf)
    let top = YGNodeLayoutGetTop(leaf)
    let width = YGNodeLayoutGetWidth(leaf)
    let height = YGNodeLayoutGetHeight(leaf)

    print("leaf \(i): origin: \(left), \(top), size: \(width), \(height)")
}

let width = YGNodeLayoutGetWidth(root)
let height = YGNodeLayoutGetHeight(root)

print("root size: \(width), \(height)")

YGNodeFreeRecursive(root)
YGConfigFree(config)
```

Pull Request resolved: #1339

Reviewed By: cipolleschi

Differential Revision: D48166958

Pulled By: NickGerleman

fbshipit-source-id: 72fef99729d01c7ba291b11f047cf75fd39f7630

.github/workflows/validate-swiftpm.yml

@@ -0,0 +1,22 @@
+name: Swift
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
+
+jobs:
+  test:
+    name: Build
+    runs-on: macos-latest
+
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Build Debug
+        run: swift build -c debug
+
+      - name: Build Release
+        run: swift build -c release

Package.swift

@@ -0,0 +1,32 @@
+// swift-tools-version:5.0
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+
+import PackageDescription
+
+let package = Package(
+    name: "yoga",
+    products: [
+        .library(name: "yoga", targets: [ "core" ])
+    ],
+    targets: [
+        .target(
+            name: "core",
+            path: ".",
+            sources: [
+                "yoga"
+            ],
+            publicHeadersPath: ".",
+            cxxSettings: [
+                .headerSearchPath(".")
+            ]
+        )
+    ],
+    cxxLanguageStandard: CXXLanguageStandard(rawValue: "c++17")
+)

yoga/module.modulemap

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+module yoga [system] {
+    module core {
+        header "YGEnums.h"
+        header "YGMacros.h"
+        header "YGValue.h"
+        header "Yoga.h"
+        export *
+    }
+}