feat(highcharts): implement subplot-grid-custom (#2876)

## Implementation: `subplot-grid-custom` - highcharts

Implements the **highcharts** version of `subplot-grid-custom`.

**File:** `plots/subplot-grid-custom/implementations/highcharts.py`

**Parent Issue:** #2856

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:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20608472991)*

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