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?

Start by reproducing the Snowflake query `SELECT v.$2 FROM (VALUES (1, 'value1'), (2, 'value2')) AS v` in the parser's Snowflake dialect handling. Trace how compound expressions process the `$2` positional parameter after an alias. Done means this valid Snowflake expression parses successfully without regressing the existing `SELECT $2 FROM ...` case.

Written by the indexing model from the issue text.