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/_meta/options.lua

@@ -1218,8 +1218,12 @@ vim.go.cia = vim.go.completeitemalign
 --- 	    completion in the preview window.  Only works in
 --- 	    combination with "menu" or "menuone".
 ---
---- Only "fuzzy", "longest", "popup", "preinsert" and "preview" have an
---- effect when 'autocomplete' is enabled.
+---     preselect   Select the completion item that has the "preselect"
+--- 	    attribute set. If both "noselect" and "preselect" are present,
+--- 	    "preselect" takes precedence.
+---
+--- Only "fuzzy", "longest", "popup", "preinsert", "preselect" and
+--- "preview" have an effect when 'autocomplete' is enabled.
 ---
 --- This option does not apply to `cmdline-completion`.  See 'wildoptions'
 --- for that.

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 = {

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,