Add support for draft/read-marker

References: https://github.com/ircv3/ircv3-specifications/pull/489

lib/client.js

@@ -1015,4 +1015,32 @@ export default class Client extends EventTarget {
 			return { message: msg.params[2] };
 		});
 	}
+
+	supportsReadMarker() {
+		return this.caps.enabled.has("draft/read-marker") || this.caps.enabled.has("soju.im/read");
+	}
+
+	_markReadCmd() {
+		if (this.caps.enabled.has("draft/read-marker")) {
+			return "MARKREAD";
+		} else if (this.caps.enabled.has("soju.im/read")) {
+			return "READ";
+		} else {
+			return null;
+		}
+	}
+
+	fetchReadMarker(target) {
+		this.send({
+			command: this._markReadCmd(),
+			params: [target],
+		});
+	}
+
+	setReadMarker(target, t) {
+		this.send({
+			command: this._markReadCmd(),
+			params: [target, "timestamp="+t],
+		});
+	}
 }