Improve lets-plot wrapper

* Set the "discrete domain" option for categorical scales (maybe use new `scaleDiscrete` method in LPK)
* Improve lets-plot custom scales internal Kandy API
* Split the scale wrapping logic into separate functions
* Add tests
* Add parameters validation
* Improve type casting logic
* Add dataset validation

The issue names no source files, tests, or entry points; start by locating the lets-plot wrapper and the LPK scale APIs, especially the new scaleDiscrete method. Done means the listed scale wrapping, casting, parameter and dataset validation improvements are implemented, with tests added for the revised behavior.

Written by the indexing model from the issue text.