Add SSR styling to reduce LCP for `amp-base-carousel` by showing first slide at first paint

### Description

When using `amp-base-carousel`, the contents are hidden until the custom element is built. This results in a negative impact on LCP. Depending on the configuration of the `amp-base-carousel`, what if it could be SSR'ed so that the initial slide were displayed even prior to the element being built? This would greatly reduce LCP by allowing the first slide to be rendered with the initial paint of the page. It would also ensure that the first slide would render even when JavaScript is turned off or the `amp-base-carousel` script fails to download.

Consider this example: https://amp-base-carousel-ssr-styling.glitch.me/

Here's a video showing the page loading without throttling a few times, then a few times on fast 3G, and then a few times on slow 3G:

https://user-images.githubusercontent.com/134745/133321141-d543257c-b5d6-44ec-8087-40b7614a04a0.mov

The styling here applied is:

```css
amp-base-carousel.with-ssr-styling:not(.i-amphtml-built) > *:first-of-type:not(i-amphtml-sizer) {
display: block !important;
color: black !important;
line-height: normal !important;
position: absolute;
top: 0;
content-visibility: visible !important;
}
```

The specific styling would surely need to be tailored to how the `amp-base-carousel` is configured.

The styling here is overriding these rules:

https://github.com/ampproject/amphtml/blob/bdfdcd0bd8ac47770982aefdddce4db0209bd942/css/ampshared.css#L284-L290

https://github.com/ampproject/amphtml/blob/bdfdcd0bd8ac47770982aefdddce4db0209bd942/css/ampshared.css#L269-L277

https://github.com/ampproject/amphtml/blob/bdfdcd0bd8ac47770982aefdddce4db0209bd942/css/ampshared.css#L304-L310

https://github.com/ampproject/amphtml/blob/bdfdcd0bd8ac47770982aefdddce4db0209bd942/css/ampshared.css#L284-L290

https://github.com/ampproject/amphtml/blob/bdfdcd0bd8ac47770982aefdddce4db0209bd942/extensions/amp-base-carousel/1.0/amp-base-carousel.css#L12-L17

Such SSR styling would likely apply to other similar components, including `amp-image-slider`.

### Alternatives Considered

n/a

### Additional Context

_No response_

Leggi le regole referenziate in css/ampshared.css e extensions/amp-base-carousel/1.0/amp-base-carousel.css, quindi confrontale con l'esempio collegato. Determina come dovrebbe variare lo stile in base alla configurazione di amp-base-carousel e verifica che la diapositiva iniziale sia visibile prima che l'elemento venga creato, anche quando JavaScript è disabilitato o lo script non riesce.

Scritto dal modello di indicizzazione a partire dal testo della issue.