Add GitHub update checker and installer

Library/Update/UpdateTrack.swift

@@ -0,0 +1,26 @@
+import Foundation
+
+public enum UpdateTrack: String, Codable, CaseIterable {
+    case stable
+    case beta
+
+    public static var defaultForCurrentBuild: Self {
+        Bundle.main.version.contains("-") ? .beta : .stable
+    }
+
+    public static func resolved(from rawValue: String) -> Self {
+        guard !rawValue.isEmpty else {
+            return defaultForCurrentBuild
+        }
+        return Self(rawValue: rawValue) ?? defaultForCurrentBuild
+    }
+
+    public func allows(_ updateInfo: UpdateInfo) -> Bool {
+        switch self {
+        case .stable:
+            return !updateInfo.isPrerelease
+        case .beta:
+            return true
+        }
+    }
+}