Should all scales expose the aesthetics parameter?

[teunbrand]

This issue is a follow-up on #2555, where all colour/fill scales exposed their aesthetics argument, to allow for non-standard aesthetics in extension packages. The question here is, should other scales, like scale_linewidth_continuous() or scale_shape_manual() similarly have aesthetics = "linewidth" and aesthetics = "shape" in their formals?

The main reason we might want to is so that it is easier for extention packages to implement 'synonyms' of aesthetics (in the same way that colour and fill are synonymous aesthetics). It might also be beneficial for PRs like #5484 to generalise beyond colours.

The decision to include non-colour scales was deferred to the future in #2555 (comment), as it would be an API change that requires some thought. I suppose that future has meanwhile arrived.