Add a method to access the list of registered colormaps #18490

Describe the issue

I noticed that the linked issues to PR
#16991
all want a public way to access the registered colormap names without resorting to importing pyplot. That PR removed the cm.cmap_d dictionary, but left nothing in its place.

There were a few suggestions in this thread: #16991 (comment)

Summary

Create a way to access the keys of the colormap registry within the mpl.cm module.

Proposed fix

Some ideas:
mpl.cm.get_cmap_names()
mpl.cm.get_registered_cmaps()
keeping consistency with pyplot: mpl.cm.colormaps()
... something else entirely