Notebook document features

[nayeemrmn]

3.17 feature: https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#notebookDocument_synchronization

The Deno LSP would use this for jupyter notebook intellisense. Specifically having continuity for variables between TypeScript cells.

cc @ebkalderon You excluded this from #255 because it was handled by #229, but that PR was closed without merge.

P.S. It would be nice to have a #[serde(flatten)] custom: Option<Map<String, Value>> field in ServerCapabilities. With tower-lsp we could get ahead of not-yet-supported request methods using LspServiceBuilder::custom_method(). But there's no way of doing that with capabilities.

[nayeemrmn]

Ah there's a PR: #268.