Export `default_expansion()` and `expand_limits_scale()` for coord extensions

I'd like to request that default_expansion() and expand_limits_scale() be exported (or that equivalent public functions be provided). These two internal helpers appear to be the only way for a coord extension to reproduce ggplot2's exact axis-expansion behavior.

I maintain ggcube, a coord extension that adds a 3D coordinate system (coord_3d()) to ggplot2. Because it extends CoordCartesian, it needs to expand scale limits identically to core ggplot2 — including taking a user-supplied expand argument — so that the z axis behaves like the x and y axes. To do this, the relevant ggcube code currently uses these internal functions by reaching into the ggplot2 namespace:

default_expansion   <- utils::getFromNamespace("default_expansion",   "ggplot2")
expand_limits_scale <- utils::getFromNamespace("expand_limits_scale", "ggplot2")

expansion      <- default_expansion(scale, expand = expand)
expanded_range <- expand_limits_scale(scale, expansion, limits, coord_limits = NULL)

Relying on these internals is fragile. If they were exported, it would fix that risk. ggplot2 already exports some adjacent helper functions, including expansion() and expand_limits(), so there seems to be a precedent.

Thanks for considering.

Start by reviewing the internal default_expansion() and expand_limits_scale() helpers alongside the already exported expansion() and expand_limits() functions. Determine whether exporting the helpers or providing equivalent public functions best supports coord extensions such as ggcube, with user-supplied expansion and matching x/y axis behavior as the completion criteria.