Cleanup io calls and add cd function.

include/internal/functions/functions.h

@@ -81,5 +81,6 @@ struct Function {
     handle;
 
   static void register_posix(BEd &ctx);
+  static void register_extented(BEd &ctx);
 };
 } // namespace bed::internal::functions

include/internal/io/io.h

@@ -74,6 +74,7 @@ struct IO {
   KeyEvent read_key();
   void write(const char *, uint64_t);
   void write(std::string_view);
+  void write_line(std::string_view);
   void run_pty(const std::string &);
 
   std::deque<char> input_queue;