[Postgres] Parsing sequence of `SHOW` and `SET` is a bit off

```rust
Parser::parse_sql(&PostgreSqlDialect {}, "SHOW search_path; SHOW timezone; SET search_path TO public;").unwrap()
```

The above results in:

```rust
[
ShowVariable {
variable: [
Ident {
value: "search_path",
quote_style: None,
span: Span(Location(1,6)..Location(1,17)),
},
Ident {
value: "SHOW",
quote_style: None,
span: Span(Location(1,19)..Location(1,23)),
},
Ident {
value: "timezone",
quote_style: None,
span: Span(Location(1,24)..Location(1,32)),
},
Ident {
value: "SET",
quote_style: None,
span: Span(Location(1,34)..Location(1,37)),
},
Ident {
value: "search_path",
quote_style: None,
span: Span(Location(1,38)..Location(1,49)),
},
Ident {
value: "TO",
quote_style: None,
span: Span(Location(1,50)..Location(1,52)),
},
Ident {
value: "public",
quote_style: None,
span: Span(Location(1,53)..Location(1,59)),
},
],
},
]
```

Its `Display` impl results in `"SHOW search_path SHOW timezone SET search_path TO public"`

使用这里所示的 Parser::parse_sql、PostgreSqlDialect 和 SHOW/SET 序列重现该问题。检查 PostgreSQL 方言如何解析这些语句，以及它们的 Display 输出是如何生成的。完成的标准是：这些语句被解析为独立的语句并显示，同时不会将后续命令吸收到第一个 SHOW 变量中。

由索引模型根据 Issue 内容生成。