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>

runtime/doc/usr_41.txt

@@ -1,4 +1,4 @@
-*usr_41.txt*	For Vim version 9.1.  Last change: 2025 Apr 21
+*usr_41.txt*	For Vim version 9.1.  Last change: 2025 Apr 24
 
 		     VIM USER MANUAL - by Bram Moolenaar
 
@@ -1124,6 +1124,8 @@ Insert mode completion:				*completion-functions*
 	complete_add()		add to found matches
 	complete_check()	check if completion should be aborted
 	complete_info()		get current completion information
+	complete_match()	get insert completion start match col and
+				trigger text
 	pumvisible()		check if the popup menu is displayed
 	pum_getpos()		position and size of popup menu if visible