Add missing schema for field widget.

Needs field widget schema for the "machine_name" widget. Missing schema file may not create a visible issue But it surely does create an issue for phpunit test cases.

Drupal\Core\Config\Schema\SchemaIncompleteException: Schema errors for core.entity_form_display.block_content.basic.default with the following errors: core.entity_form_display.block_content.basic.default:content.machine_name.settings.source missing schema, core.entity_form_display.block_content.basic.default:content.machine_name.settings.exists missing schema, core.entity_form_display.block_content.basic.default:content.machine_name.settings.size missing schema, core.entity_form_display.block_content.basic.default:content.machine_name.settings.placeholder missing schema, core.entity_form_display.block_content.basic.default:content.machine_name.settings.label missing schema, core.entity_form_display.block_content.basic.default:content.machine_name.settings.replace_pattern missing schema, core.entity_form_display.block_content.basic.default:content.machine_name.settings.replace missing schema