Add chiptune synth instrument

[n1LS]

Implement a new synth instrument with the following goals:

• chiptune-focused (LSDJ-esque)
• performant enough to allow 8 voices on picoTracker
• support all relevant commands

[n1LS]

Demo song to test the instrument: chiptune.zip

[maks]

@n1LS as a starting point, could you provide here a more detailed explanation and description of the synth, the kind of text copy that would go into the user manual for this new synth instrument (which I note is missing from your draft PR).

[n1LS]

There is no documentation yet as you asked to submit the instrument as draft PR for early review. I will get to that soon.

[maks]

Sure can start looking at the code but would really help to know how it's supposed to work when looking at the code to see if it matches that.

[n1LS]

@maks There's some documentation now :-)