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

Problem:
Unnecessary C impl of default 'titlestring'.

Solutin:
Define it using format flags.

runtime/doc/vim_diff.txt

@@ -616,6 +616,11 @@ Autocommands:
 - |TermResponse| is fired for any OSC sequence received from the terminal,
   instead of the Primary Device Attributes response. |v:termresponse|
 
+Options:
+- 'titlestring' uses printf-style '%' items (see: 'statusline') to implement
+  the default behaviour. The implementation is equivalent to setting
+  'titlestring' to `%t%(\ %M%)%(\ \(%{expand(\"%:~:h\")}\)%)%a\ -\ Nvim`.
+
 ==============================================================================
 Missing features                                         *nvim-missing*