Add type definition for beforeEvent changed

[joshkel]

I'm not certain whether supporting args.changed from beforeEvent is intended or not:

• It works.
• It wasn't in the type definitions.
• The migration guide only references it for afterEvent.
• The plugin documentation does NOT limit it to afterEvent...
• ...although the plugin documentation's flowchart puts the notice next to afterEvent.
• chartjs-plugin-annotation's draggable sample depends on using it from beforeEvent.

In the case that it should be supported, this PR adds it to the type definitions.