Add support for LIMIT prepared query

Hi, I am using sqlparser 0.4.0 and rust version 1.36.0

Sql Query
`
SELECT "emp"."id" FROM "emp" WHERE "emp"."id" = $1 LIMIT $2 -- binds: [123, 5]
`

Parse error
`
ParserError("Expected literal int, found: $2")
`

Used Dialect:
```rust
impl Dialect for AckoPostgresSqlDialect {
fn is_identifier_start(&self, ch: char) -> bool {
(ch >= 'a' && ch <= 'z')
|| (ch >= 'A' && ch <= 'Z')
|| (ch == '@')
|| ch == '$'
|| ch == '_'
}

fn is_identifier_part(&self, ch: char) -> bool {
(ch >= 'a' && ch <= 'z')
|| (ch >= 'A' && ch <= 'Z')
|| (ch >= '0' && ch <= '9')
|| (ch == '@')
|| ch == '$'
|| ch == '_'
}
}
```

从所示查询的 LIMIT 解析路径入手，检查 $2 在提供的 AckoPostgresSqlDialect 下是如何分类的。为 LIMIT 中的预处理参数添加支持，然后验证示例查询是否能使用 binds [123, 5] 成功解析。

由索引模型根据 Issue 内容生成。