Add "broken" fallback plugin for mapping field types which can't be loaded

modules/salesforce_mapping/src/Form/MappedObjectForm.php

...lesforce_mapping/src/Form/SalesforceMappingFieldsForm.php

...salesforce_mapping/src/Form/SalesforceMappingFormBase.php

...sforce_mapping/src/Form/SalesforceMappingFormCrudBase.php

...orce_mapping/src/Plugin/SalesforceMappingField/Broken.php

...force_mapping/src/Plugin/SalesforceMappingField/Token.php

...rce_mapping/src/SalesforceMappingFieldPluginInterface.php

...force_mapping/src/SalesforceMappingFieldPluginManager.php