Added details of where to create post type classes to "Can't load" message.
Updated references to "plugin" to "core" as now used with themes as well.
Fixed issue with registering custom post types to themes

Updated ::register() method to type hint using Base rather than Plugin, as could also be a Theme object.
My attempt at merging in the upstream changes (which didn't work!)
Merge branch 'master' into ncf-version

  Fixed issue with registering custom post types to themes
  Updated references to "plugin" to "core" as now used with themes as well.
  Added details of where to create post type classes to "Can't load" message.
Reverted pluginFile back to coreFile, as should be type agnostic.

Think this may need refactoring though, as not sure that the theme version needs to reference it's core file.