Add reaction filter

Closes #715

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

@@ -44,6 +44,12 @@ internal static class FilterGrammar
         .Select(v => (MessageFilter) new MentionsMessageFilter(v))
         .Named("mentions:<value>");
 
+    private static readonly TextParser<MessageFilter> ReactionFilter = Span
+        .EqualToIgnoreCase("reaction:")
+        .IgnoreThen(String)
+        .Select(v => (MessageFilter) new ReactionMessageFilter(v))
+        .Named("reaction:<value>");
+
     private static readonly TextParser<MessageFilter> HasFilter = Span
         .EqualToIgnoreCase("has:")
         .IgnoreThen(Parse.OneOf(
@@ -72,6 +78,7 @@ internal static class FilterGrammar
         GroupedFilter,
         FromFilter,
         MentionsFilter,
+        ReactionFilter,
         HasFilter,
         ContainsFilter
     );

DiscordChatExporter.Core/Exporting/Filtering/ReactionMessageFilter.cs

@@ -0,0 +1,18 @@
+using System;
+using System.Linq;
+using DiscordChatExporter.Core.Discord.Data;
+
+namespace DiscordChatExporter.Core.Exporting.Filtering;
+
+internal class ReactionMessageFilter : MessageFilter
+{
+    private readonly string _value;
+
+    public ReactionMessageFilter(string value) => _value = value;
+
+    public override bool IsMatch(Message message) => message.Reactions.Any(r =>
+        string.Equals(_value, r.Emoji.Id?.ToString(), StringComparison.OrdinalIgnoreCase) ||
+        string.Equals(_value, r.Emoji.Name, StringComparison.OrdinalIgnoreCase) ||
+        string.Equals(_value, r.Emoji.Code, StringComparison.OrdinalIgnoreCase)
+    );
+}