refactor!: warn of future branch_separator change

lua/persisted/init.lua

@@ -82,6 +82,16 @@ function M.setup(opts)
   config.setup(opts)
   setup_commands()
 
+  if not opts.branch_separator and config.options.branch_separator == "_" then
+    vim.notify(
+      "[Persisted.nvim] We will soon be changing the default `branch_separator` config value to `@@`\n" ..
+      "[Persisted.nvim] We notice you're using the current default value of `_`\n" ..
+      "[Persisted.nvim] To continue using this, please add `branch_separator = \"_\"` to your config and we'll quit bugging you\n" ..
+      "[Persisted.nvim] Otherwise, please replace the `_` in your session names with `@@`",
+      vim.log.levels.WARN
+    )
+  end
+
   if config.options.autoload and (allow_dir() and not ignore_dir()) and vim.fn.argc() == 0 then
     M.load()
   end