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/eval.lua

@@ -1477,6 +1477,59 @@ M.funcs = {
     returns = 'table',
     signature = 'complete_info([{what}])',
   },
+  complete_match = {
+    args = { 0, 2 },
+    base = 0,
+    desc = [=[
+      Returns a List of matches found according to the 'isexpand'
+      option. Each match is represented as a List containing
+      [startcol, trigger_text] where:
+      - startcol: column position where completion should start,
+        or -1 if no trigger position is found. For multi-character
+        triggers, returns the column of the first character.
+      - trigger_text: the matching trigger string from 'isexpand',
+        or empty string if no match was found or when using the
+        default 'iskeyword' pattern.
+
+      When 'isexpand' is empty, uses the 'iskeyword' pattern
+      "\k\+$" to find the start of the current keyword.
+
+      When no arguments are provided, uses the current cursor
+      position.
+
+      Examples: >
+        set isexpand=.,->,/,/*,abc
+        func CustomComplete()
+        let res = complete_match()
+        if res->len() == 0 | return | endif
+        let [col, trigger] = res[0]
+        let items = []
+        if trigger == '/*'
+          let items = ['/** */']
+        elseif trigger == '/'
+          let items = ['/*! */', '// TODO:', '// fixme:']
+        elseif trigger == '.'
+          let items = ['length()']
+        elseif trigger =~ '^\->'
+          let items = ['map()', 'reduce()']
+        elseif trigger =~ '^\abc'
+          let items = ['def', 'ghk']
+        endif
+        if items->len() > 0
+          let startcol = trigger =~ '^/' ? col : col + len(trigger)
+          call complete(startcol, items)
+        endif
+        endfunc
+        inoremap <Tab> <Cmd>call CustomComplete()<CR>
+<
+
+      Return type: list<list<any>>
+    ]=],
+    name = 'complete_match',
+    params = { { 'lnum', 'integer?' }, { 'col', 'integer?' } },
+    returns = 'table',
+    signature = 'complete_match([{lnum}, {col}])',
+  },
   confirm = {
     args = { 1, 4 },
     base = 1,