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: https://github.com/facebook/yoga/pull/1339

Reviewed By: cipolleschi

Differential Revision: D48166958

Pulled By: NickGerleman

fbshipit-source-id: 72fef99729d01c7ba291b11f047cf75fd39f7630

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 *
+    }
+}