Add has:pin filter (#853)

Adds a new `has:*` filter. Specifically, `has:pin`.
The filter filters for messages that are pinned.

DiscordChatExporter.Core/Exporting/Filtering/HasMessageFilter.cs

@@ -19,6 +19,7 @@ internal class HasMessageFilter : MessageFilter
         MessageContentMatchKind.Video => message.Attachments.Any(file => file.IsVideo),
         MessageContentMatchKind.Image => message.Attachments.Any(file => file.IsImage),
         MessageContentMatchKind.Sound => message.Attachments.Any(file => file.IsAudio),
+        MessageContentMatchKind.Pin => message.IsPinned,
         _ => throw new InvalidOperationException($"Unknown message content match kind '{_kind}'.")
     };
 }

DiscordChatExporter.Core/Exporting/Filtering/MessageContentMatchKind.cs

@@ -7,5 +7,6 @@ internal enum MessageContentMatchKind
     File,
     Video,
     Image,
-    Sound
+    Sound,
+    Pin
 }

DiscordChatExporter.Core/Exporting/Filtering/Parsing/FilterGrammar.cs

@@ -58,7 +58,8 @@ internal static class FilterGrammar
             Span.EqualToIgnoreCase("file").IgnoreThen(Parse.Return(MessageContentMatchKind.File)),
             Span.EqualToIgnoreCase("video").IgnoreThen(Parse.Return(MessageContentMatchKind.Video)),
             Span.EqualToIgnoreCase("image").IgnoreThen(Parse.Return(MessageContentMatchKind.Image)),
-            Span.EqualToIgnoreCase("sound").IgnoreThen(Parse.Return(MessageContentMatchKind.Sound))
+            Span.EqualToIgnoreCase("sound").IgnoreThen(Parse.Return(MessageContentMatchKind.Sound)),
+            Span.EqualToIgnoreCase("pin").IgnoreThen(Parse.Return(MessageContentMatchKind.Pin))
         ))
         .Select(k => (MessageFilter) new HasMessageFilter(k))
         .Named("has:<value>");