[react-native-navigation] Make navigator a class
[react-native-navigation] Make navigator a class

Navigator is exported by rnn and is a class.
cc @egorshulga
laurentiustamate94/DefinitelyTyped,arusakov/DefinitelyTyped,AgentME/DefinitelyTyped,mcliment/DefinitelyTyped,magny/DefinitelyTyped,chrootsu/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,AgentME/DefinitelyTyped,magny/DefinitelyTyped,markogresak/DefinitelyTyped,borisyankov/DefinitelyTyped,georgemarshall/DefinitelyTyped,arusakov/DefinitelyTyped,AgentME/DefinitelyTyped,georgemarshall/DefinitelyTyped,AgentME/DefinitelyTyped,dsebastien/DefinitelyTyped,arusakov/DefinitelyTyped,one-pieces/DefinitelyTyped,chrootsu/DefinitelyTyped,rolandzwaga/DefinitelyTyped,borisyankov/DefinitelyTyped,dsebastien/DefinitelyTyped,rolandzwaga/DefinitelyTyped