“Next closest accessible color” feature

## Description

Feature in npm module and web tool to return next closest accessible color(s)

User inputs color, tool returns 3:1, 4.5:1, and 7:1 variations.

## Why do you need this feature?

Simplified use case to support end user theming of white label design systems, as well as providing designers with simplified experience for recommendation of “next best color”

Some assumptions may need to be made, or leaving open the api for interpolation mode/types as not all color recommendations would be best in any specific interpolation mode. My also need exploration if alternative curves or “projections” should be modeled so recommendation can be made based on human aesthetics (although this may be out of scope)

## Leonardo package and version

## Additional context

Start by reviewing the npm module and web tool entry points to understand how colors and contrast ratios are represented. Define the supported interpolation or projection behavior and verify that a color input produces 3:1, 4.5:1, and 7:1 recommendations; the feature is done when both interfaces return consistent results.

Written by the indexing model from the issue text.