feat(postgre): #412 update merge's syntax and with clause syntax

Author: zhaoge

src/grammar/postgresql/PostgreSqlParser.g4

@@ -1911,11 +1911,13 @@ prep_type_clause
     : OPEN_PAREN typename (COMMA typename)* CLOSE_PAREN
     ;
 
+// https://www.postgresql.org/docs/17/sql-select.html
 preparablestmt
     : selectstmt
     | insertstmt
     | updatestmt
     | deletestmt
+    | mergestmt
     ;
 
 executestmt
@@ -3558,8 +3560,10 @@ stmt_move
     : KW_MOVE opt_fetch_direction? cursor_variable SEMI
     ;
 
+// https://www.postgresql.org/docs/17/sql-merge.html
 mergestmt
     : with_clause? KW_MERGE KW_INTO (KW_ONLY)? table_name (STAR)? (KW_AS? colid)? KW_USING data_source KW_ON expression merge_when_clause+
+        returning_clause?
     ;
 
 data_source