Add fill area between series support to XY Chart (fillBelowTo)

**Why is this needed**: The XY Chart visualization currently doesn't support filling areas between series (similar to the `fillBelowTo` option available in Time Series panels). This limitation prevents users from creating proper control charts, confidence intervals, and range visualizations when the X-axis is not time-based. **What would you like to be added**: Add fill area support to XY Chart with the following field override properties: - `custom.fillOpacity` - Control the transparency of the fill area (0-100) - `custom.fillBelowTo` - Specify which series to fill toward This would provide feature parity with Time Series panel but for non-temporal X-axis data. Example configuration: ```json { "matcher": { "id": "byName", "options": "Serie A" }, "properties": [ { "id": "custom.fillBelowTo", "value": "Serie B" }, { "id": "custom.fillOpacity", "value": 25 } ] } ``` **Who is this feature for?** This feature is for users working with: - Manufacturing and quality control teams creating Statistical Process Control charts - Data analysts visualizing ranges, confidence intervals, or tolerance bands with non-temporal data - Engineers monitoring process parameters against specification limits - Anyone needing to display area fills in scatter plots or XY relationships