add field assignment to statements

src/main/antlr4/de/hsrm/compiler/Klang/Klang.g4

@@ -40,6 +40,7 @@ statement
   | if_statement
   | variable_declaration SCOL
   | variable_assignment SCOL
+  | field_assignment SCOL
   | return_statement
   | whileLoop
   | doWhileLoop
@@ -68,6 +69,10 @@ variable_assignment
   : IDENT EQUAL expression
   ;
 
+field_assignment
+  : IDENT (DOT IDENT)+ EQUAL expression
+  ;
+
 return_statement
   : RETURN expression SCOL
   ;