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: #16716

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/structs.h

@@ -3302,6 +3302,7 @@ struct file_buffer
     char_u	*b_p_fo;	// 'formatoptions'
     char_u	*b_p_flp;	// 'formatlistpat'
     int		b_p_inf;	// 'infercase'
+    char_u	*b_p_ise;	// 'isexpand' local value
     char_u	*b_p_isk;	// 'iskeyword'
 #ifdef FEAT_FIND_ID
     char_u	*b_p_def;	// 'define' local value