These have been replaced in v3 with a new spec.
As we will soon migrate which-key to `settings` options, I've named the
new option `settings.spec` so that we do not need to "rename" it again
soon.
This commit **does not** actually add a freeform settings option.
* plugins/which-key: add registration support for key groups
Modified the type of registration to support registration of key groups with "groupkey" = { name = "name";};.
* plugins/which-key: relax the restrition on the type of registrations and more tests have been added
* plugins/which-key: add more tests for groups within groups
