Add ability to read/write "standard" MIDI files.

[duhrer]

This ticket covers adding support for reading and writing "standard" MIDI files. This would support:

1. Capturing, reusing, and transforming performed MIDI messages.
2. Saving things like system exclusive messages required to configure devices to external files and loading them in code.
3. Accessing the wealth of historical examples of music saved in this format.
4. Generating synthetic tracks for use with external devices and programs (for example, "Black MIDI" compositions played using Synthesia).

This work would be added in a separate part of the youme namespace (for example youme.smf) and would be "opt in", i.e. authors would have to reference that file to add support in their work. There would be a separate page of documentation linked from the main docs.

My thinking is that the initial implementation probably won't include the later extensions to the standard such as karaoke files, but the work here should be tested with those files if possible to confirm that they load what they can without error.

I did review "prior art", including jzz-midi-smf, smfplayer, picoaudio.js. Each of these provides a parser coupled with a player, and redefines many of the concepts we already have for dealing with MIDI messages. It seemed more appropriate to extend what we have, i.e. we need new mechanisms for reading/writing things like headers and tracks, but can reuse the functions we have to read/write MIDI messages.

As part of that research, I found the test-midi-files package provided by JazzSoft under an MIT License. Although that package provides a player as well, the player is not required to use the test files, and the test files cover many things I would like to verify support for.