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>

runtime/lua/vim/_meta/vimfn.lua

@@ -1109,6 +1109,55 @@ function vim.fn.complete_check() end
 --- @return table
 function vim.fn.complete_info(what) end
 
+--- 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>>
+---
+--- @param lnum? integer?
+--- @param col? integer?
+--- @return table
+function vim.fn.complete_match(lnum, col) end
+
 --- confirm() offers the user a dialog, from which a choice can be
 --- made.  It returns the number of the choice.  For the first
 --- choice this is 1.