Add protocol for serial library to implement.

Sources/SwiftRPLidar/LidarSerial.swift

@@ -0,0 +1,8 @@
+protocol LidarSerial {
+    var dtr: Bool {get set};
+    var inWaiting: Int {get};
+    func closePort() -> Void;
+    func openPort() -> Void;
+    readData(ofLength: Int) throws -> Void;
+    writeData(_ data: Data) throws -> Void;
+}