Kernel: Perform output processing on echo

Previously, we would echo characters back just as they were passed to
us, even in canonical mode. This caused newlines to not work correctly
in some programs.

Fixes #7802

Kernel/TTY/TTY.h

@@ -80,6 +80,10 @@ protected:
 private:
     // ^CharacterDevice
     virtual bool is_tty() const final override { return true; }
+    inline void echo_with_processing(u8);
+
+    template<typename Functor>
+    void process_output(u8, Functor put_char);
 
     CircularDeque<u8, TTY_BUFFER_SIZE> m_input_buffer;
     // FIXME: use something like AK::Bitmap but which takes a size template parameter