`insertReplaceSupport` LSP client capability should be supported

`insertReplaceSupport` is a client capability add to LSP in 3.16.0. It allows a client to report to server whether it intends the server to insert text or replace some text on completion. rust-analyzer is a server that [defaults to replace](https://github.com/rust-lang/rust-analyzer/issues/21365) if such capability isn't reported, but will respect a client's choice. For this particular case, having support for this capability allows us to work around https://github.com/prabirshrestha/vim-lsp/issues/1625. I've created a PR: https://github.com/prabirshrestha/vim-lsp/pull/1630