Potential focus visibility issue in annotations/hotspots

## Potential a11y issue

Annotations may fail WCAG [Focus Visible](https://www.w3.org/WAI/WCAG22/Understanding/focus-visible.html) and/or [Focus Appearance](https://www.w3.org/WAI/WCAG22/Understanding/focus-appearance) on focus.

### Steps to reproduce:

1. Go to https://modelviewer.dev/examples/annotations/#cameraViews
2. Tab to the first annotation/hotspot
3. Notice how the focused annotation element and belonging focus outline is barely visible:

### Relevant CSS:

https://github.com/google/model-viewer/blob/297ed2bdbea0c8f921d985ff0c71afd3a819e12e/packages/model-viewer/src/template.ts#L106-L108

## Potential solution

We can ensure the element and focus outline is visible on focus by not changing the opacity if there's [focus-within](https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Selectors/:focus-within) the slotted annotation wrapper:

```css
.annotation-wrapper.hide ::slotted(:not(:focus-within)) {
opacity: var(--min-hotspot-opacity, 0.25);
}
```

### Result:

The annotation and focus outline is now fully visible if there's focus within:

## Disclaimer

I'm not personally a user of ``, I'm here mostly due to @svinkle's model a11y work and my interest in [the `` element proposal](https://github.com/immersive-web/model-element).

Start with packages/model-viewer/src/template.ts around lines 106-108 and reproduce the issue at modelviewer.dev/examples/annotations/#cameraViews by tabbing to the first annotation. Adjust the focus-related visibility behavior described in the issue, then verify that the annotation and its focus outline remain clearly visible when focus is within the slotted wrapper.

Written by the indexing model from the issue text.