[Feature] Ability to exponentially scale the size of a node in Sankey charts

### What problem does this feature solve?

Imagine a Sankey chart like this where the value of a node is disproportionally bigger than other nodes.


The chart looks accurate but it is not useful to see the relative differences between nodes. A more useful way to show this data would be to scale the data exponentially and render it like that:


### What does the proposed API look like?

The Sankey series should support `exponent: number` option and its default value should be 1. It should exponentiate all values with this number before calculating the layout.

Locate the Sankey series option definition and the layout calculation entry point. Verify how node values are transformed before layout, then confirm that an exponent option defaults to 1, changes the rendered proportions when set, and preserves current behavior at 1.

Written by the indexing model from the issue text.