feat(highcharts): implement contour-density (#2595)

## Implementation: `contour-density` - highcharts

Implements the **highcharts** version of `contour-density`.

**File:** `plots/contour-density/implementations/highcharts.py`

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:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20593358403)*

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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]