branch: soundDesignCourse

[jbphet]

This branch is being set up as a place to work on variations of the sound design behavior needed for a sound design course that is being developed by @briannatomlinson. For now, the only variation that has been requested is one where the cooling sound is on by default and it's the only sound played. Issue https://github.com/phetsims/phet-io-wrapper-sonification/issues/92 is sort of the "parent issue" for this one, where all the needed sim variants are listed.

[jbphet]

I've made the changes for this and have published a version with only the "cooling off" sound at https://phet-dev.colorado.edu/html/friction/1.6.0-soundDesignCourse.1/phet/friction_en_phet.html.

[jbphet]

Assigning to myself and marking as deferred for now, since no action is needed until the sound design course is done or I get feedback that something different is needed.