Add banner to UserUpdateBuilder, and SkuFlags.available (#638)

* add banner to UserUpdateBuilder, and SkuFlags.available

* add UserUpdateBuilder.banner test

lib/src/builders/user.dart

@@ -1,16 +1,24 @@
 import 'package:nyxx/src/builders/builder.dart';
 import 'package:nyxx/src/builders/image.dart';
+import 'package:nyxx/src/builders/sentinels.dart';
 import 'package:nyxx/src/models/user/user.dart';
 
 class UserUpdateBuilder extends UpdateBuilder<User> {
+  /// New user's username.
   String? username;
+
+  /// New user's avatar.
   ImageBuilder? avatar;
 
-  UserUpdateBuilder({this.username, this.avatar});
+  /// New user's banner.
+  ImageBuilder? banner;
+
+  UserUpdateBuilder({this.username, this.avatar = sentinelImageBuilder, this.banner = sentinelImageBuilder});
 
   @override
   Map<String, Object?> build() => {
         if (username != null) 'username': username!,
-        if (avatar != null) 'avatar': avatar!.buildDataString(),
+        if (!identical(avatar, sentinelImageBuilder)) 'avatar': avatar?.buildDataString(),
+        if (!identical(banner, sentinelImageBuilder)) 'banner': banner?.buildDataString(),
       };
 }

lib/src/models/sku.dart

@@ -68,6 +68,9 @@ enum SkuType {
 
 /// Flags applied to an [Sku].
 class SkuFlags extends Flags<SkuFlags> {
+  /// The SKU is available for purchase.
+  static const available = Flag<SkuFlags>.fromOffset(0);
+
   /// The SKU is a guild subscription.
   static const guildSubscription = Flag<SkuFlags>.fromOffset(7);
 
@@ -77,6 +80,9 @@ class SkuFlags extends Flags<SkuFlags> {
   /// Create a new [SkuFlags].
   SkuFlags(super.value);
 
+  /// Whether this set of flags has the [available] flag set.
+  bool get isAvailable => has(available);
+
   /// Whether this set of flags has the [guildSubscription] flag set.
   bool get isGuildSubscription => has(guildSubscription);
 

test/unit/builders/user_test.dart

@@ -21,6 +21,9 @@ void main() {
 
       final builder4 = UserUpdateBuilder();
       expect(builder4.build(), equals({}));
+
+      final builder5 = UserUpdateBuilder(banner: ImageBuilder.png([]));
+      expect(builder5.build(), equals({'banner': 'data:image/png;base64,'}));
     });
   });
 }