Become more robust to parse errors

I've started working on a sql [lsp server implementation](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/), using `sqlparser` as the parser. Unfortunately, I very quickly hit a wall, mostly around how `sqlparser` discovers and surfaces errors. For instance, let's say the user types the following: `SELECT * F` and hits the key. A good lsp server would suggest completing `F` to `FROM`, even just do it automatically in that case. If one tries to parse that input string with `sqlparser`, it returns a `ParserError("Expected end of statement, found: F")`. The error isn't unexpected, but now (a) since the ast isn't available in the face of errors, there are no recommendations/commands/lints I can make based on a error-prone AST, and (b) I don't have enough information (line/col info) to send a diagnostic (the thing that makes the red squiggly lines) to the client. The line/col information was suggested in #179, but a fair amount of the work that would enable a really good LSP server implementation is being able to work with an AST that has errors in it.

I _suspect_ what I'm describing is _really_ hard, and has all sorts of backwards-incompatible changes that would be required to make this work. I think it'd be worthwhile, though.

まず、sqlparser の ParserError 処理と、`SELECT * F` のパース時に関係する parser のエントリポイントを読みます。issue には具体的なファイルやテストの指定はありません。現在の動作を LSP の診断要件と比較し、エラー許容 AST、completion のサポート、行/列の診断をどのように定義すべきかを決定します。Done には、合意した動作と、この例のような不完全な SQL に対するカバレッジを含める必要があります。

索引モデルが issue の本文から書いたものです。