[Feature] add missing invite guild properties & welcome screen support (#2510)

* added models

* working getter for welcome screen

* <see langword="null"/>

* more changes

* modify welcome screen support

* fix some typos & remove `using` added by VS

* Working-ish state

* Resolve some reviews

* change access modifier

* forgot to add docs

* revert to InviteGuild & extend it

* resolve some reviews

* Apply suggestions from code review

Co-authored-by: Quin Lynch <49576606+quinchs@users.noreply.github.com>

Co-authored-by: Cenk Ergen <57065323+Cenngo@users.noreply.github.com>
Co-authored-by: Quin Lynch <49576606+quinchs@users.noreply.github.com>

src/Discord.Net.Rest/Entities/Guilds/RestGuild.cs

@@ -1534,6 +1534,15 @@ namespace Discord.Rest
             else
                 return null;
         }
+
+        /// <inheritdoc/>
+        public Task<WelcomeScreen> GetWelcomeScreenAsync(RequestOptions options = null)
+            => GuildHelper.GetWelcomeScreenAsync(this, Discord, options);
+
+        /// <inheritdoc/>
+        public Task<WelcomeScreen> ModifyWelcomeScreenAsync(bool enabled, WelcomeScreenChannelProperties[] channels, string description = null, RequestOptions options = null)
+            => GuildHelper.ModifyWelcomeScreenAsync(enabled, description, channels, this, Discord, options);
+
         #endregion
     }
 }