Import Jenkins WorkflowScript class
I think this class is build dynamically to attach into it all available plugin extensions. But I think you can use super class for WorkflowScript - CpsScript:
import org.jenkinsci.plugins.workflow.cps.CpsScriptMore information about the CpsScript can be found here: https://javadoc.jenkins.io/plugin/workflow-cps/org/jenkinsci/plugins/workflow/cps/package-tree.html