feat(lsp): add a Language Server Protocol server

[nokome]

As a VSCode user (like, actually, IRL) I would like to be able to use Stencila's functionality from within VSCode. See the discussion about creating a Stencila VSCode extension. Users of other code editors such a NeoVim and Zed might also want to use Stencila from within their favorite editor.

All the above editors are clients to the Language Server Protocol. By implementing a LSP server for Stencila we can enable things like completions, hover etc.

Resources

• https://code.visualstudio.com/api/language-extensions/language-server-extension-guide
• https://microsoft.github.io/language-server-protocol/

Notes

• Issue feat(markdown codec): return position-to-node mapping for from_str #2175 needs to be completed before this can really be useful because it will provide the mapping between Markdown source positions and nodes
• https://github.com/oxalica/async-lsp (my preliminary research suggests this might be the best crate to use, it compares itself the other two options in the README)

[nokome]

Closing as this is done