onchange on Wizard cmp needs renamed to be more explicit

[mtchevalier]

The onchange event handler on <c-wizard> catches change events from nested components like <lightning-input> unless I explicitly call event.preventPropogation() on the event handler the input component.

The wizard framework should be modified so that this event/event handler combo uses a more explicit naming structure that has less chance of crossing over with standard component events.

Something like onwizardstepchange would work just fine