refactor(options): impl default 'titlestring' by format flags #30843

Problem:
Unnecessary C impl of default 'titlestring'.

Solutin:
Define it using format flags.

src/nvim/options.lua

@@ -9157,6 +9157,10 @@ return {
         expanded according to the rules used for 'statusline'.  If it contains
         an invalid '%' format, the value is used as-is and no error or warning
         will be given when the value is set.
+
+        The default behaviour is equivalent to: >vim
+            set titlestring=%t%(\ %M%)%(\ \(%{expand(\"%:~:h\")}\)%)%a\ -\ Nvim
+        <
         This option cannot be set in a modeline when 'modelineexpr' is off.
 
         Example: >vim