TelnetServer: Implement basic telnet server

Fixes #407

Depends on #530 to run reliably.

Servers/TelnetServer/Parser.h

@@ -0,0 +1,33 @@
+#pragma once
+
+#include <AK/Function.h>
+#include <AK/String.h>
+#include <AK/StringView.h>
+#include <AK/Types.h>
+
+#include "Command.h"
+
+#define IAC 0xff
+
+class Parser {
+public:
+    Function<void(const Command&)> on_command;
+    Function<void(const StringView&)> on_data;
+    Function<void()> on_error;
+
+    void write(const StringView&);
+
+protected:
+    enum State {
+        Free,
+        ReadCommand,
+        ReadSubcommand,
+        Error,
+    };
+
+    void write(const String& str);
+
+private:
+    State m_state { State::Free };
+    u8 m_command { 0 };
+};