Cannot parse named windows with modifiers

sqlparser-rs only supports named windows that are an identifier. This is fine for queries like:

```sql
SELECT sum(salary) OVER w, avg(salary) OVER w
FROM empsalary
WINDOW w AS (PARTITION BY depname ORDER BY salary DESC);
```

However, it is insufficient for queries in at least BigQuery and MySQL where modifiers can be added to the named window in the OVER clause.

[MySQL named window example](https://dev.mysql.com/doc/refman/8.0/en/window-functions-named-windows.html)

```sql
SELECT
DISTINCT year, country,
FIRST_VALUE(year) OVER (w ORDER BY year ASC) AS first,
FIRST_VALUE(year) OVER (w ORDER BY year DESC) AS last
FROM sales
WINDOW w AS (PARTITION BY country)
```

[BigQuery named window example](https://cloud.google.com/bigquery/docs/reference/standard-sql/window-function-calls)

```sql
SELECT item, purchases, category, LAST_VALUE(item)
OVER (
item_window
ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING
) AS most_popular
FROM Produce
WINDOW item_window AS (
PARTITION BY category
ORDER BY purchases)
```

This would build upon prior to work:
- https://github.com/sqlparser-rs/sqlparser-rs/issues/867
- https://github.com/sqlparser-rs/sqlparser-rs/pull/881

まず、sqlparser-rs が OVER 句内の named-window 式をどのようにパースするかを調べ、次に以前の設計について issue #867 と pull request #881 を確認してください。MySQL と BigQuery の例に対するパースのカバレッジを追加し、修飾子付きの named windows が、通常の named windows に回帰を起こさずに受け入れられることを検証してください。

索引モデルが issue の本文から書いたものです。