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/completion.lua

@@ -441,6 +441,7 @@ function M._lsp_to_complete_items(
         empty = 1,
         abbr_hlgroup = hl_group,
         kind_hlgroup = kind_hlgroup,
+        preselect = item.preselect,
         user_data = {
           nvim = {
             lsp = {