Support providing an image for progressive enhancement

[spaceninja]

@Paul-Hebert suggested:

From a progressive enhancement perspective it would be kind of nice if the poster image was a slotted img tag but I realize this would diverge from the model-viewer API so I understand if that goes against your goals.

<lite-model-viewer>
  <img alt="" src="" slot="poster" />
</lite-model-viewer>

[spaceninja]

This is a slick idea — if we added it, I think we'd update the JS to check for an img in the "poster" slot, and grab the src value to apply as the poster.

I only hesitated to pull the trigger on this because it's unclear what value a non-JS experience provides here, given that <model-viewer> itself is JS-only.