amp-story-interactive-slider variant for expressing opinion relative to a central/neutral position

### Description

I would like to suggest a slider component that is designed to represent opinion on a `-100 > 0 > 100` or `-50 > 0 > 50` scale.
There are many questions/topics which fit on this scale e.g. (political left/right).

I think it needs two key features:

1) Configure where on the scale the slider starts i.e. in the middle.
2) Do not increase the size of the emoji/percentage towards the right hand end of the scale, but instead increase the further you are from the center/default start position.

### Alternatives Considered

The slider component is clearly designed for a scale of `0 > 100`, rather than `-100 > 0 > 100`.

### Additional Context

_No response_

Start by examining the amp-story-interactive-slider component and its existing 0–100 behavior. Trace how the initial slider position and emoji/percentage sizing are configured. Done means the component can represent a negative-to-positive scale with a configurable neutral starting position and scales visual feedback by distance from that neutral point.

Written by the indexing model from the issue text.