Missing propagation of the progress event reason

[Ryan Leyland]

Description

PR #4647 added a reason to progress events. Unfortunately this reason is not being propagated properly in loading.ts.
The reason needs to be passed to the new CustomEvent() in loading.ts in order to be accessible in the ModelViewerElement.
Currently we have:

Which should be more like:

this.dispatchEvent(
          new CustomEvent('progress', {detail: {totalProgress: progress, reason}}));

Version

• model-viewer: v3.4.0

Browser Affected

• Chrome, version: xx.x.xxxx.xx
• Edge
• Firefox
• IE
• Safari

OS

• Android
• iOS
• Linux
• MacOS
• Windows

AR

• WebXR
• SceneViewer
• QuickLook