Fix "original" scale limits with nonlinear pan

[joshkel]

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.

I wanted JSDoc + TypeScript checks to help validate and document these changes, but that exposed apparent limitations of Chart.js core's type definitions. I'll open a separate PR for those.

[trullock]

This is merged into my fork at https://github.com/trullock/chartjs-plugin-zoom which is published on npm as @trullock/chartjs-plugin-zoom, as this fork seems abandoned