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/buffer.c b/src/buffer.c
index 0624f9d..697efa3 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2494,6 +2494,7 @@
     clear_string_option(&buf->b_p_cinw);
     clear_string_option(&buf->b_p_cot);
     clear_string_option(&buf->b_p_cpt);
+    clear_string_option(&buf->b_p_ise);
 #ifdef FEAT_COMPL_FUNC
     clear_string_option(&buf->b_p_cfu);
     free_callback(&buf->b_cfu_cb);