add AutoSession.session_exists_for_cwd() function

Add a function to understand if a session exists for the given current
working directory.

This is pretty helpful for start page plugins like mini.starter and
others.

By calling this function when nvim is opened to a workspace folder the
starter page can determine whether to show a 'restore session' option or
not.

Signed-off-by: ldelossa <louis.delos@gmail.com>

lua/auto-session/init.lua

@@ -370,6 +370,14 @@ local function auto_save_conditions_met()
   return is_enabled() and auto_save() and not suppress_session() and is_allowed_dir() and not bypass_save_by_filetype()
 end
 
+-- Quickly checks if a session file exists for the current working directory.
+-- This is useful for starter plugins which don't want to display 'restore session'
+-- unless a session for the current working directory exists.
+function AutoSession.session_exists_for_cwd()
+	session_file = get_session_file_name(vim.fn.getcwd())
+	return Lib.is_readable(session_file)
+end
+
 ---AutoSaveSession
 ---Function called by auto_session to trigger auto_saving sessions, for example on VimExit events.
 ---@param sessions_dir? string the session directory to auto_save a session for. If empty this function will end up using the cwd to infer what session to save for.