Support of new Syntax

Hello,

a project of the company I am working for, requires a high performance SQL parser such as this one. However, we would mainly require the [Exasol](https://www.exasol.com/) syntax (which is a lot like Oracle syntax) and we may also need DB2 (z/OS and LUW) syntax.

We are considering and willing to implement the needed syntax and would love to officially contribute that syntax here and share it with the community. But before we start working on it, we have to check our internal policies, whether we are allowed to do so or not. Furthermore, we would like to ask you as well for your opinion on this, since Exasol is not a very popular or widespread RDBMS and the readme specifically states the following:
> The most commonly accepted PRs add support for ... a **popular** RDBMS ... will likely be accepted after a brief review.

We want to make sure, that our contribution would be be accepted and supported (being reviewed) by the responsible contributors of this project (of course only if it meets all the standards and contribution guidelines).

I am looking forward to a hopefully positive reply!

この issue にはファイル、テスト、エントリーポイントが記載されていません。まず README の構文サポートとコントリビューションに関するガイダンスを読み、その後、Exasol と DB2 のスコープについてメンテナーと話し合う前に、現在ダイアレクトがどのように表現されているかを確認してください。Done にするには、合意されたスコープと承認済みの実装計画が必要であり、単なるパーサーの変更では不十分です。

索引モデルが issue の本文から書いたものです。