Implement DotNet callback functions

[bmotmans]

[JSInvokable("FormatLabel")]
private string FormatLabel(dynamic item)
{
	return $"{item.Name} ({item.percent * 2}%)";
}

Challenges:

• A lot of variations to support: instance <> static, Task<T> vs T vs dynamic return type
• There are functions with multiple parameters: we need a way to detect this dynamically

[bmotmans]

Also needs support for custom events: https://echarts.apache.org/examples/en/editor.html?c=geo-seatmap-flight

myChart.on('geoselectchanged', function (params) {
    const selectedNames = params.allSelected[0].name.slice();
    // Remove taken seats.
    for (var i = selectedNames.length - 1; i >= 0; i--) {
      if (takenSeatNames.indexOf(selectedNames[i]) >= 0) {
        selectedNames.splice(i, 1);
      }
    }
    console.log('selected', selectedNames);
  });