Trying to use Makie with the new Pkg extension feature in Julia 1.9

Observations:

• You cannot export a function defined in an extension (or at least you're not supposed to as far as I know). So I defined two empty functions someplot and someplot! in SomePackage and then imported them in the extension. This caused @recipe(SomePlot)... to add its methods to those functions. A little tricky maybe, but could be workable.
• To guide unknowing users, I've implemented an error hint that only fires for MethodErrors on the plotting functions if they don't yet have any methods, and tells users that Makie needs to be loaded.
• Using Requires, the extension is made usable on pre 1.9 systems as well.