Change `ar` attribute configuration to `unstable-webxr` about model-viewer HOT 4 CLOSED

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Echoing discussion in #61 (comment)

I think it's fine to pick any given default. However, I also think it's important to give the content author control over what is actually shown to the end user. They may not want a "present in AR" button in some contexts or at all. A boolean configuration is an easy way to enable this flexibility.

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Related to this topic, the ar attribute might be better as an ar-controls attribute. Instead of thinking of it as enabling or disabling AR altogether, we should think of it as enabling or disabling the built-in controls for launching AR.

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I think that "AR" (WebXR, ML, iOS, etc.) should be the default, and if for some reason, someone's visiting this content on Helio, Chrome Canary with WebXR flags, or on iOS with a USDZ provided, which is already a terribly small subset of users, it "just works" without having to opt in. This is part of the "works out of the box" story, rather than needing to explicitly enable, or worse, enable for different AR platforms (would have to keep up to date on what is supported, and update content accordingly). If a developer wanted to explicitly disable AR, we could provide a disable-ar attribute, or disable-{webxr|magicleap} for individual platforms.

We should make this work by default with AR, and have an attribute for the exceptional case. Thoughts?

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