Feature Request: Implement pull model diagnostics

This is not a bug, but a feature request. I would like vim-language-server to implement pull model diagnostics, which is a new feature in the LSP spec that I proposed initially. See here: https://github.com/microsoft/language-server-protocol/issues/737 See the LSP spec for more information here: https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/

If the pull model is implemented in the Vim language server, it will later work with ALE, so that people won't see warnings or errors in their files until an ALE lint cycle happens, instead of at random.