Line chart - combination of fill `start` and `end`

### Feature Proposal

Hello all,

I'm working an a react-app displaying a pretty complex line chart.

Is there a possibility to combine both `fill:start` and `fill:end` properties, for the data point area to be filled above and under the data point entry?

My current dataset config looks like this:

```
export const initialUnderMinimum: ChartDataset<'line'> = {
label: 'UNDER_MIN_VALUE',
data: [],
showLine: true,
fill: 'start',
}
```

The chart gets rendered in a following manner:

The data gets filled dynamically after the application start based upon a reoccuring GET-request.

The only way to achieve what i want, is to create the copy of the dataset with fill: 'end' and fill it with data the same as i did with the first dataset.
That is, however, not very efficient if the data has like 5000 entries.

Basically i want to achieve something like this:

`chart.js`-Version is 4.2.1, `react-chartjs-2` version is 5.2.0.

Feel free to close the issue, if that's the case for the other ( `react-chartjs-2`) package.

### Possible Implementation

Implement a feature to be able to pass an array or function as the fill value.