feat(lua): add notify_once() (#17010)

Like vim.notify(), but only displays the notification once.

This function prints a warning message to the user only once per Nvim
session. This is useful for things we want the user to see without being
overwhelmed with warning messages (for example, the deprecation messages
in LSP diagnostics).

runtime/lua/vim/lsp/util.lua

@@ -10,14 +10,6 @@ local uv = vim.loop
 local npcall = vim.F.npcall
 local split = vim.split
 
-local _warned = {}
-local warn_once = function(message)
-  if not _warned[message] then
-    vim.api.nvim_err_writeln(message)
-    _warned[message] = true
-  end
-end
-
 local M = {}
 
 local default_border = {
@@ -1933,7 +1925,6 @@ function M.lookup_section(settings, section)
 end
 
 M._get_line_byte_from_position = get_line_byte_from_position
-M._warn_once = warn_once
 
 M.buf_versions = {}