Export Music XML #30

Effectively Scribe consumes Sequence JSON, splits it into measures, parts and symbols, and spits out HTML. Making it spit out Music XML instead would be trivial.

The time-consuming part will be making sure that it gets the structure of the enormous list of Music XML elements right:

https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/

Should this be part of Scribe, or a module in the Sequence repo? Probably ultimately a module in the Sequence repo, but the obvious first step to getting it working is to modify Scribe to output Music XML.