The new buffer update API is not going to work as it limits to
MessagePack-RPC plugins.
- Structured, independent, reproducible tests.
- More integration tests.
-- Register buffer modified notification instead of TextChanged autocmd. Wait
- for https://github.com/neovim/neovim/pull/5269 to be merged in.
- Check server capabilities before calling RPC API.
- Replace echodoc functionality.
