move SMS to its own file

 ## NEXT
 Improvements:
-* The `type` of an `Email` is no longer null.
+* The `type` of an `Email` is now non-null.
+* The `phoneNumber` of an `SMS` is now non-null.
 
 ## 3.5.0
 New Features:

@@ -18,4 +18,5 @@ export 'src/objects/calendar_event.dart';
 export 'src/objects/driver_license.dart';
 export 'src/objects/email.dart';
 export 'src/objects/mobile_scanner_arguments.dart';
+export 'src/objects/sms.dart';
 export 'src/objects/wifi.dart';

@@ -9,6 +9,7 @@ import 'package:mobile_scanner/src/enums/phone_type.dart';
 import 'package:mobile_scanner/src/objects/calendar_event.dart';
 import 'package:mobile_scanner/src/objects/driver_license.dart';
 import 'package:mobile_scanner/src/objects/email.dart';
+import 'package:mobile_scanner/src/objects/sms.dart';
 import 'package:mobile_scanner/src/objects/wifi.dart';
 
 /// Represents a single recognized barcode and its value.
@@ -274,24 +275,6 @@ class Phone {
         type = PhoneType.values[data['type'] as int];
 }
 
-/// A sms message from a 'SMS:' or similar QRCode type.
-class SMS {
-  /// Gets the message content of the sms.
-  ///
-  /// Returns null if not available.
-  final String? message;
-
-  /// Gets the phone number of the sms.
-  ///
-  /// Returns null if not available.
-  final String? phoneNumber;
-
-  /// Create a [SMS] from native data.
-  SMS.fromNative(Map data)
-      : message = data['message'] as String?,
-        phoneNumber = data['phoneNumber'] as String?;
-}
-
 /// A URL and title from a 'MEBKM:' or similar QRCode type.
 class UrlBookmark {
   /// Gets the title of the bookmark.

+/// An sms message from a `SMS:` or similar QRCode type.
+class SMS {
+  /// Construct a new [SMS] instance.
+  const SMS({
+    this.message,
+    required this.phoneNumber,
+  });
+
+  /// Construct a new [SMS] instance from the given [data].
+  factory SMS.fromNative(Map<Object?, Object?> data) {
+    return SMS(
+      message: data['message'] as String?,
+      phoneNumber: data['phoneNumber'] as String? ?? '',
+    );
+  }
+
+  /// The message contained in the sms.
+  final String? message;
+
+  /// The phone number which sent the sms.
+  final String phoneNumber;
+}