vim-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: vim/vim#16716

bcd5995b40

Co-authored-by: glepnir <glephunter@gmail.com>

src/nvim/options.lua

@@ -4628,6 +4628,29 @@ local options = {
       type = 'boolean',
       immutable = true,
     },
+    {
+      abbreviation = 'ise',
+      cb = 'did_set_isexpand',
+      defaults = '',
+      deny_duplicates = false,
+      desc = [=[
+        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|.
+      ]=],
+      full_name = 'isexpand',
+      list = 'onecomma',
+      scope = { 'global', 'buf' },
+      short_desc = N_('Defines characters and patterns for completion in insert mode'),
+      type = 'string',
+      varname = 'p_ise',
+    },
     {
       abbreviation = 'isf',
       cb = 'did_set_isopt',