[Linux] Support MIDI playback

[nmlgc]

Won't be part of the main Linux porting work in #42 because the whole point of #9 was that we wouldn't have to do this. Still, a lack of MIDI means that the Linux port will play no music until you set up a BGM pack.

Then again, due to the general lack of a default MIDI synth on Linux, a Linux-supporting MIDI backend can never provide a plug-and-play solution with zero tinkering. (Unless you connect a hardware synth, and there certainly are people who'd like to do that even under Linux.)

Will probably involve a library like libremidi to abstract away ALSA/JACK/PipeWire, and even the CoreMIDI and Web MIDI APIs of a hypothetical macOS or Emscripten port.

If you plan to fund this, please tell me why:

• Do you just want to connect a hardware synth or FluidSynth?
• Or do you want a similar plug-and-play experience like on Windows, offering some sort of cheap MIDI rendering as a fallback without having to download a BGM pack? This might still be worthwhile if we consider that only 3 of the 16 YouTube channels that uploaded Shuusou Gyoku videos between P0275 and P0303 bothered to set up a BGM pack.