add fontSize property to label-series

[dandgerson]

Thank you for amazing library. It very useful in our production.

I want to add some functional to our label-series
I have to some customize fontSize property for labels and suggest this feature.

I had this small PR as fist step. I would be glad to continue commit

Example:

<LabelSeries
          animation={animation}
          className={className}
          rotation={labelRotation}
          labelAnchorY="text-before-edge"
          data={nodesCopy.map((node, i) => {
            return {
              x: node.x0 + (node.x0 < width / 2 ? nWidth + 10 : -10),
              y: (node.y0 + node.y1) / 2 - marginTop,
              label: node.name,
              style: {
                textAnchor: node.x0 < width / 2 ? 'start' : 'end',
                dy: '-.5em',
                fontSize: node.fontSize || 'inherit', // add fontSize
                ...style.labels
              },
              // unfortunately this can not be ...node as the version
              // found in nodesCopy is modified by the sankey process
              ...nodes[i]
            };
          })}
        />

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