irc: Strip lone backslashes from tag values

According to the message-tags spec:

> If a lone \ exists at the end of an escaped value (with no escape
> character following it), then there SHOULD be no output character. For
> example, the escaped value test\ should unescape to test.

https://ircv3.net/specs/extensions/message-tags

assets/irc.js

@@ -36,6 +36,9 @@ function parseTags(s) {
 		for (var ch in tagsEscape) {
 			v = v.replaceAll(tagsEscape[ch], ch);
 		}
+		if (v.endsWith("\\")) {
+			v = v.slice(0, v.length - 1)
+		}
 		tags[k] = v;
 	});
 	return tags;