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/todo.txt

@@ -1,4 +1,4 @@
-*todo.txt*      For Vim version 9.1.  Last change: 2025 Mar 27
+*todo.txt*      For Vim version 9.1.  Last change: 2025 Apr 24
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -4749,20 +4749,10 @@ Insert mode completion/expansion:
 7   When expanding file names with an environment variable, add the match with
     the unexpanded var.  So $HOME/tm expands to "/home/guy/tmp" and
     "$HOME/tmp"
-8   When there is no word before the cursor but something like "sys." complete
-    with "sys.".  Works well for C and similar languages.
 9   ^X^L completion doesn't repeat correctly.  It uses the first match with
     the last added line, instead of continuing where the last match ended.
     (Webb)
-8   Add option to set different behavior for Insert mode completion:
-    - ignore/match case
-    - different characters than 'iskeyword'
-8   Add option 'isexpand', containing characters when doing expansion (so that
-    "." and "\" can be included, without changing 'iskeyword'). (Goldfarb)
-    Also: 'istagword': characters used for CTRL-].
-    When 'isexpand' or 'istagword' are empty, use 'iskeyword'.
-    Alternative: Use a pattern so that start and end of a keyword can be
-    defined, only allow dash in the middle, etc.
+8   Add option 'istagword': characters used for CTRL-]. like 'isexpand'
 8   Add a command to undo the completion, go back to the original text.
 7   Completion of an abbreviation: Can leave letters out, like what Instant
     text does: www.textware.com