Allow <picture> element

### Description

[With `amp-img` deprecated](https://github.com/ampproject/amphtml/issues/30442), allow HTML's `picture` element too.

This would be useful so that those who use `img` over `amp-img` have a way to use different sources/fallback images without having to resort back to using `amp-img`.

### Alternatives Considered

Improve `amp-img` fallback to allow it to safely contain multiple fallback images of different image types without browsers downloading more than one working image. For example, I find that Microsoft Edge browser will unnecessarily download multiple fallback images from `amp-img` even though the first image worked.

### Additional Context

_No response_