Add Emscripten build to the CI suite

[rpatters1]

A powerful feature of this library (and the musx -> musicxml library I am building on top of it) is the ability to compile it with emscripten and use it client-side in a browser from a website.

If the CI is going to be refactored for Windows, I think including an emscripten build is something to consider as well. Right now denigma is building for emscripten in a separate repo, but I may add that test to the main repo at some point.

Edit: promoted to #386

[rpatters1]

For grins I add a musicxml conversion wasm project (compiling mx with emscripten) here.

[webern]

Oh cool. Yeah this is an interesting idea. If I understand it correctly, it works right now, the gate would just be to prevent compile errors in the future?

[rpatters1]

That is correct. It works now. The main caveat is you have to compile with exceptions enabled, because emscripten defaults to no exceptions. And there is no access to the file system. My demo project uses a node script to test that it can take a musx file and turn it into a musicxml string.

[webern]

When you first created this discussion, I didn't quite understand it because I figured without JavaScript bindings, what good is it. Now I see what you are doing. All of the transformation and processing is happening in the C++ code, so you don't need a WASM interface to mx, you just need it in your runtime. I'll take another look at adding a build to keep this working in CI.