Add sustain pedal to PC keyboard

[tatsuki-makino]

As reported in #197, the sound that starts when the sustain pedal is on will continue to sound until the sustain pedal is turned off.
The sound is not decayed in any way, so if the sustain pedal is lost while it is on, there is no way to stop the sound.
So, I used the following patch to assign the keys on the keyboard to the operation of the sustain pedal.
patch.txt
The N key on the keyboard is the key for it.
This allows the sustain to be released, but since it is a momentary operation, there is a disadvantage that it takes precedence over the side that requires sustain.