These currently raise spurious errors, due to limitations in Chart.js types.

Nonlinear scales failed to resolve "original" within panNumericalScale.

To fix this:

* Resolving "original" within panNumericalScale seemed redundant, when
  updateRange was already doing it, so I instead moved the logic into
  updateRange and control it with a new special value for the `zoom`
  parameter.  (Hopefully the use of a special string like this is
  acceptable as long as JSDoc calls it out - if I should use another
  approach, please let me know.)
* This change put updateRange over the configured ESLint complexity
  limit, so I extracted a new getScaleLimits function to reduce
  complexity.

Add unit tests, both for this bug and for the preexisting but untested
"original" feature.