add `combine_aes()` function à la ggforce

[EvaMaeRey]

Given, ggforce's filter inspiration #6774, another add that might be useful to the extender crowd could be combine_aes(), helpful in that it afford greater concision and also help better record which part of existing the defaults are the target of a change, as is done for example in GeomCircle's definition:

GeomCircle <- ggproto('GeomCircle', GeomShape,
  default_aes = combine_aes(GeomShape$default_aes, aes(colour = 'black', fill = NA))
)

[teunbrand]

Thanks for the suggestion Gina! I can see why it is useful, but I don't see why it has to live in ggplot2. It is a tiny function with no additional dependencies on internals that folks can simply copy over to their package if they need it (ggforce uses MIT licence) .

[EvaMaeRey]

yeah. I think my thinking is more around what's modeled in the extension documentation - I like a pathway that kind of records your intent for cases where you're gonna borrow most defaults, for example GeomPoint, I just want to change the shape - to 21, for example and keep everything else the same. My go to is utils::modifyList for this actually, but wonder if a different drop-in would be better. modifyList's keep.null = FALSE default seems like it could be problematic.