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>

src/evalfunc.c

@@ -2104,6 +2104,8 @@ static funcentry_T global_functions[] =
 			ret_number_bool,    f_complete_check},
     {"complete_info",	0, 1, FEARG_1,	    arg1_list_string,
 			ret_dict_any,	    f_complete_info},
+    {"complete_match",	0, 2, 0,	    NULL,
+			ret_list_any,	    f_complete_match},
     {"confirm",		1, 4, FEARG_1,	    arg4_string_string_number_string,
 			ret_number,	    f_confirm},
     {"copy",		1, 1, FEARG_1,	    NULL,