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>
diff --git a/src/option.h b/src/option.h
index db1030d..bb1226e 100644
--- a/src/option.h
+++ b/src/option.h
@@ -731,6 +731,7 @@
 EXTERN char_u	*p_indk;	// 'indentkeys'
 #endif
 EXTERN int	p_im;		// 'insertmode'
+EXTERN char_u	*p_ise;		// 'isexpand'
 EXTERN char_u	*p_isf;		// 'isfname'
 EXTERN char_u	*p_isi;		// 'isident'
 EXTERN char_u	*p_isk;		// 'iskeyword'
@@ -1205,6 +1206,7 @@
     , BV_INEX
 #endif
     , BV_INF
+    , BV_ISE
     , BV_ISK
 #ifdef FEAT_CRYPT
     , BV_KEY