Addresses #3283
Rather than using custom UI for showing the loaded state. Rely
on the WorkDoneProgress API in 3.15.0
https://microsoft.github.io/language-server-protocol/specification#workDoneProgress.
No client-side work was necessary. The UI is not exactly what is
described in the issue but afaict that's how VS Code implements the LSP
API.
- The WorkDoneProgressEnd does not appear to display its message
contents (controlled by vscode)
083035fd06