lint: rename IsStream to IsStreaming

While it might be more proper to refer to a voice state as composing
a stream (i.e., the IVoiceState is a Stream), this property is ported
up most often to actual user entities. It makes more sense to
communicate that the user is streaming, rather than stating that the
user itself is a stream.

This is also more gramatically consistent with the other voice state
properties (e.g., IsMuted). Movement from noun-form to participle-form
remains inline with the past-participle form used on the other
properties.

src/Discord.Net.Rest/Entities/Users/RestGroupUser.cs

@@ -36,6 +36,6 @@ namespace Discord.Rest
         /// <inheritdoc />
         string IVoiceState.VoiceSessionId => null;
         /// <inheritdoc />
-        bool IVoiceState.IsStream => false;
+        bool IVoiceState.IsStreaming => false;
     }
 }

src/Discord.Net.Rest/Entities/Users/RestGuildUser.cs

@@ -152,6 +152,6 @@ namespace Discord.Rest
         /// <inheritdoc />
         string IVoiceState.VoiceSessionId => null;
         /// <inheritdoc />
-        bool IVoiceState.IsStream => false;
+        bool IVoiceState.IsStreaming => false;
     }
 }

src/Discord.Net.Rest/Entities/Users/RestWebhookUser.cs

@@ -96,6 +96,6 @@ namespace Discord.Rest
         /// <inheritdoc />
         string IVoiceState.VoiceSessionId => null;
         /// <inheritdoc />
-        bool IVoiceState.IsStream => false;
+        bool IVoiceState.IsStreaming => false;
     }
 }