To see the demo, start a web server and load this page (e.g., `npx http-server` and navigate to http://127.0.0.1:8080/demos/fonts/core-with-promise.html).
Using vexflow-core.js allows your webpage to load a small initial bundle. It will load fonts on demand when
you call Vex.Flow.fetchMusicFont(fontName)
. This method is asynchronous and returns a Promise.
Open your browser's developer tools to see that font modules are downloaded upon request:
View the page's source.