feat(postgresql): add columnref in target_el directly (#23)

In Bytebase query span, we need to handling star expansion like SELECT 
table.* FROM table. Considering returning SourceColumnSet for other 
candidates in a_expr make code cleaner.

Author: h3n4l

postgresql/PostgreSQLParser.g4

@@ -4030,7 +4030,8 @@ target_list
    ;
 
 target_el
-   : a_expr target_alias? # target_label
+   : columnref # target_columnref  // NOTE(parser): Add by Bytebase to handling table.* easily.
+   | a_expr target_alias? # target_label
    | STAR # target_star
    ;