Refactor proxy handling code

- Move reaction handlers to the ReactionAdded event instead of
  ProxyService
- Split tag matching off into ProxyTagParser
- Split autoproxy matching off into Autoproxier
- General cleanup and simplification

PluralKit.Bot/Proxy/ProxyMatch.cs

@@ -0,0 +1,25 @@
+#nullable enable
+using PluralKit.Core;
+
+namespace PluralKit.Bot
+{
+    public struct ProxyMatch
+    {
+        public PKMember Member;
+        public string? Content;
+        public ProxyTag? ProxyTags;
+
+        public string? ProxyContent
+        {
+            get
+            {
+                // Add the proxy tags into the proxied message if that option is enabled
+                // Also check if the member has any proxy tags - some cases autoproxy can return a member with no tags
+                if (Member.KeepProxy && Content != null && ProxyTags != null)
+                    return $"{ProxyTags.Value.Prefix}{Content}{ProxyTags.Value.Suffix}";
+
+                return Content;
+            }
+        }
+    }
+}