Can't parse valid Snowflake compound expression

Hey there, thanks for the great project!

I'm encountering an issue with the following query, which is valid in Snowflake:

```sql
SELECT v.$2 FROM (VALUES (1, 'value1'), (2, 'value2')) AS v
```

It can't get parsed with sqlparser-rs:

```
Expected: an identifier or a '*' after '.', found: $2 at Line: 1, Column: 10
```

While this syntax isn't valid in PostgreSQL, it is accepted by Snowflake. `SELECT $2 FROM ...` (without the alias) parses correctly, so it seems the parser has trouble with positional parameters when they're part of compound expressions.

Is this a gap in the Snowflake dialect implementation?

まず、Snowflake 方言に対応する parser の処理で、Snowflake クエリ `SELECT v.$2 FROM (VALUES (1, 'value1'), (2, 'value2')) AS v` を再現します。複合式が alias の後の位置パラメーター `$2` をどのように処理するかを追跡します。既存の `SELECT $2 FROM ...` のケースを後退させることなく、この有効な Snowflake 式が正常にパースできれば完了です。

索引モデルが issue の本文から書いたものです。