[amp-img] Support media features or dark mode

### Description

Without AMP, a dark mode version of an image can be set via `` like this:

```html

```

However, the `amp-img` component doesn't seem to have an option like that right now.

Maybe it could work in a similar way to the [fallback image](https://amp.dev/documentation/components/websites/amp-img#specify-a-fallback-image):

```html

```

### Alternatives Considered

Allow `media` to work like this:

```html

```

### Additional Context

_No response_

Start by reading the amp-img component documentation, especially the existing fallback-image behavior, and compare it with the proposed nested amp-img and media examples. Determine which API should support dark mode or general media features, then define completion as documented, working behavior for the selected proposal without breaking the current fallback behavior.

Written by the indexing model from the issue text.