feat(swarmplot): add support for log axis and legends

[tkonopka]

Addresses themes raised in #1939 and #1992

Adds features to the swarmplot:

• support for log scales along the value axis
• support for legends

Example

Implementation

Support for log scales required tweaks to the typescript definition for the valueScale prop. This was simple conceptually, but also required propagating the changes to functions that process coordinates. In practice, this is achieved by defining new types for the value-scale spec and the scale object. (Using dedicated types seemed more robust that extending the existing approach of joining primitive types using 'or'). Propoagating these changes introduces several code edits. However, the edits do not introduce any new real features. (It was already possible to use a log scale before). The advantage of the changes is that typescript users will be able to use log scales without seeing warnings.

Support for legends required addition of new props legendLabel and legends to the SwarmPlot and SwarmPlotCanvas components. These follow the nomenclature and formats from other nivo packages.

Two new stories have examples for how to use log scales and legends.

Can make further changes and improvements if needed.

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