ggmapinset 0.5.0

• coerce_centre() is a new extension point needed by {ggautomap}.
• The inset param of layer helpers and geom_sf_inset() no longer accepts NA.
  Instead waiver() is used as the default, and NULL still disables the inset.
  This should be updated in any extensions using build_sf_inset_layers().
• geom_sf_inset() no longer draws the inset if inset = waiver() but there
  is no inset configured on the coord.
• We now correctly declare the minimum supported R version.