vim-patch:9.1.1341: cannot define completion triggers

Problem:  Cannot define completion triggers and act upon it
Solution: add the new option 'isexpand' and add the complete_match()
          function to return the completion matches according to the
          'isexpand' setting (glepnir)

Currently, completion trigger position is determined solely by the
'iskeyword' pattern (\k\+$), which causes issues when users need
different completion behaviors - such as triggering after '/' for
comments or '.' for methods. Modifying 'iskeyword' to include these
characters has undesirable side effects on other Vim functionality that
relies on keyword definitions.

Introduce a new buffer-local option 'isexpand' that allows specifying
different completion triggers and add the complete_match() function that
finds the appropriate start column for completion based on these
triggers, scanning backwards from cursor position.

This separation of concerns allows customized completion behavior
without affecting iskeyword-dependent features. The option's
buffer-local nature enables per-filetype completion triggers.

closes: vim/vim#16716

bcd5995b40

Co-authored-by: glepnir <glephunter@gmail.com>

src/nvim/buffer_defs.h

@@ -558,6 +558,7 @@ struct file_buffer {
   char *b_p_fo;                 ///< 'formatoptions'
   char *b_p_flp;                ///< 'formatlistpat'
   int b_p_inf;                  ///< 'infercase'
+  char *b_p_ise;                ///< 'isexpand'
   char *b_p_isk;                ///< 'iskeyword'
   char *b_p_def;                ///< 'define' local value
   char *b_p_inc;                ///< 'include'