Fallback for `check_subclass()`

[teunbrand]

This PR aims to progress #5962 and #3798.

Taking the deprecation of GeomCol (#3798) as an example, we want users to be able to use layer(geom = "col", ...) even after the class is deprecated. This PR ensures that layer(geom = "col") finds the geom_col() constructor and returns the geom field from the resulting layer (even if we remove the GeomCol class).