Adds VS App Center analytics + crash reporting

Currently tracks install, refresh, and update app events.

AltStore/Extensions/FileManager+DirectorySize.swift

@@ -0,0 +1,36 @@
+//
+//  FileManager+DirectorySize.swift
+//  AltStore
+//
+//  Created by Riley Testut on 3/31/20.
+//  Copyright © 2020 Riley Testut. All rights reserved.
+//
+
+import Foundation
+
+extension FileManager
+{
+    func directorySize(at directoryURL: URL) -> Int?
+    {
+        guard let enumerator = FileManager.default.enumerator(at: directoryURL, includingPropertiesForKeys: [.fileSizeKey]) else { return nil }
+        
+        var total: Int = 0
+        
+        for case let fileURL as URL in enumerator
+        {
+            do
+            {
+                let resourceValues = try fileURL.resourceValues(forKeys: [.fileSizeKey])
+                guard let fileSize = resourceValues.fileSize else { continue }
+                
+                total += fileSize
+            }
+            catch
+            {
+                print("Failed to read file size for item: \(fileURL).", error)
+            }
+        }
+        
+        return total
+    }
+}