feat(completion): completeopt=preselect, LSP CompletionItem.preselect #36613

Problem: 
LSP CompletionItem.preselect is not supported.
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#completionClientCapabilities

Solution:
- Add "preselect" field to complete-items and "preselect" flag
  to 'completeopt'.
- Set preselectSupport=true in LSP client capabilities.

runtime/lua/vim/lsp/protocol.lua

@@ -474,7 +474,7 @@ function protocol.make_client_capabilities()
         completionItem = {
           snippetSupport = true,
           commitCharactersSupport = false,
-          preselectSupport = false,
+          preselectSupport = true,
           deprecatedSupport = true,
           documentationFormat = { constants.MarkupKind.Markdown, constants.MarkupKind.PlainText },
           insertReplaceSupport = true,