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/options.txt

@@ -1,4 +1,4 @@
-*options.txt*	For Vim version 9.1.  Last change: 2025 Apr 19
+*options.txt*	For Vim version 9.1.  Last change: 2025 Apr 24
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -4983,6 +4983,19 @@ A jump table for the options with a short description can be found at |Q_op|.
 	and there is a letter before it, the completed part is made uppercase.
 	With 'noinfercase' the match is used as-is.
 
+						*'isexpand'* *'ise'*
+'isexpand' 'ise'	string	(default: "")
+			local to buffer
+	Defines characters and patterns for completion in insert mode. Used by
+	the |complete_match()| function to determine the starting position for
+	completion. This is a comma-separated list of triggers. Each trigger
+	can be:
+	- A single character like "." or "/"
+	- A sequence of characters like "->", "/*", or "/**"
+
+	Note: Use "\\," to add a literal comma as trigger character, see
+	|option-backslash|.
+
 			*'insertmode'* *'im'* *'noinsertmode'* *'noim'*
 'insertmode' 'im'	boolean	(default off)
 			global