We don't act as a language server for these files (e.g. don't get open/close
notifications for them), but just blindly publish diagnostics for them.
This works reasonably well in coc.nvim and vscode: they show up in the
workspace diagnostic list and when you open the file.
The only update after the file is reparsed, not as you type which is a bit
janky, but seems a lot better than nothing.