Add support for base fields

Hi!

Thanks very much for creating this module, I really like the idea and there's a lot of work been done already on it.

I had a go with the fix you added here, and although it prevented the crash it did still have one or two issues. I've fixed these and added a small fix to the src/Plugin/Field/FieldWidget/Bert.php class that was uncovered after fixing the original issue.

I have pushed the fix to the fork that you created, so I've added a merge request as well. The fork was a little out of date with the main 2.x branch so I merged that in too.

The site I am working with now has user autocomplete fields working nicely for both the core fields and extra fields added.

I was doing some testing on this today and realised that if a bundle with an entity is added to the field then the page crashes, I've pushed a fix for that so it works with non-bundle and bundle entities.

Could you clarify why exactly the changes in Bert.php are necessary? I just tested only the changes to bert.module and I seem to be able to change the node author widget to bert, change the author of a node and save it. What issues did you encounter exactly and how can I reproduce them?

I'm not sure about this code, creating an entity using a random bundle from the allowed target bundles:

$entityBundles = $items->getItemDefinition()['settings']['handler_settings']['target_bundles'];
$entityBundle = reset($entityBundles);

It's interesting you should say that the code in Bert.php doesn't seem to be needed. I wonder if it's a side effect of broken configuration that I've just worked around on my local setup?

Let me do some investigation and check that.