fix: disambiguate BigQuery AS STRUCT/VALUE qualifier from select tail

Making the select-items group optional for the target-less PostgreSQL
SELECT let JavaCC see K_AS in the follow set of the optional AS
STRUCT/VALUE qualifier block, producing a choice conflict at the block.
Guard it with an explicit lookahead so AS only opens the qualifier when
STRUCT or VALUE comes next.

Author: princejain756

src/main/jjtree/net/sf/jsqlparser/parser/JSqlParserCC.jjt

@@ -8974,7 +8974,7 @@ PlainSelect PlainSelect() #PlainSelect:
         }
     }
 
-    [
+    [ LOOKAHEAD(<K_AS> (<K_STRUCT> | <K_VALUE>))
         <K_AS>
         (
             <K_STRUCT> { plainSelect.setBigQuerySelectQualifier( PlainSelect.BigQuerySelectQualifier.AS_STRUCT ); }