Add support for closing labels

[natebosch]

This is not part of the LSP spec, but there is an extension in the Dart server.

https://github.com/dart-lang/sdk/blob/0732bac32cd6638495055685db576aded6e33b70/pkg/analysis_server/tool/lsp_spec/README.md#darttextdocumentpublishclosinglabels-notification

It should be possible to use these in nvim with the nvim_buf_set_virtual_text method.

I don't think there is a feasible approach in vim.