Add ANTLR grammar for greetings and farewells with corresponding parser and listener implementations

Hello.g4

@@ -1,5 +1,10 @@
 grammar Hello;
 
+greeting : greet+;
+leaving : bye+;
+greet : 'hello' | 'hi' ;
+
 start :  'hello' ID ;
 ID : [a-zA-Z]+ ;
 WS : [ \t\r\n]+ -> skip ;
+bye : ('bye'| 'goodbye') ID ;