Fix / Create schema definitions and appropriate tests for all (sub)modules

Alright, these are quite a big chunk of changes, but this will ensure the module's formatter / widget settings schema to be valid at all times.

Please review! I still need to test the update hooks, but they should work. We updated formatter settings similar in photoswipe before.

Happy reviewing!

All update hooks work as expected!

Interestingly enough, I found a bug in config inspector. The config inspector still shows schema errors for the icon widget "packages" setting, even after resaving the widget settings / creating a new field and adjusting the widget settings. This doesn't make sense as

1. The new tests do NOT throw schema errors anymore after the new schema update.
2. We are using the EXACT SAME "packages" setting for a widget in our "micon_link" submodule with the EXACT SAME schema, but there we do not get any schema errors from config inspector
3. The same checkboxes form element + schema combination is used in the "book" core module. See here and here.

I think it might have something to do with the name of the formatter.

Ok, that is fascinating. It really is the name! If we change the name of the widget, all schema errors are gone!

I think this is related to the fact, that the widget has the same id as the field_type AND the formatter. And somehow Drupal core's schema validation works as intended, but the config inspector UI doesn't.

I'll create a follow-up issue for that and a config_inspector bug report.