serverless: improve Plugin type
serverless: improve Plugin type

georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,magny/DefinitelyTyped,georgemarshall/DefinitelyTyped,magny/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,borisyankov/DefinitelyTyped,borisyankov/DefinitelyTyped,mcliment/DefinitelyTyped,dsebastien/DefinitelyTyped,markogresak/DefinitelyTyped