Refactor out merge and color helpers

[benmccann]

This change better organizes the code and will allow making ./core/core.helpers importable. Making the build entirely importable will allow us to utilize optional dependencies in rollup (example), which will enable users to use moment or luxon, etc.

After this change, the methods in ./core/core.helpers can start to be moved into the helpers directory so that they are included in ./helpers/index. This change is necessary to prevent circular dependencies when making that improvement (see newly added code comment for explanation)

[kurkle]

I tend to agree with @simonbrunel #5959 (comment):

For example configMerge and scaleMerge are not "helpers" but complex logic proper to our main controller and should certainly be moved in core/core.controller.js with a deprecated aliases.

color OTOH could be useful externally, maybe even in its own namespace?

[benmccann]

I think @simonbrunel is going to take a stab at implementing this, so I'll close this PR for now. @simonbrunel hopefully this PR helps illuminate the possible circular dependency issue you might run into and one possible solution. Hope it helps