Fix return type for cd, rm, mv, mkdir, ln, chmod, touch to ShellString
Fix return type for cd, rm, mv, mkdir, ln, chmod, touch to ShellString

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