vim-patch:9.1.1933: completion: complete_match() is not useful (#36726)

Problem:  completion: complete_match() Vim script function and
          'isexpand' option are not that useful and confusing
          (after v9.1.1341)
Solution: Remove function and option and clean up code and documentation
          (Girish Palya).

complete_match() and 'isexpand' add no real functionality to Vim. They
duplicate what `strridx()` already does, yet pretend to be part of the
completion system. They have nothing to do with the completion mechanism.

* `f_complete_match()` in `insexpand.c` does not call any completion code.
   It’s just a `STRNCMP()` wrapper with fluff logic.
* `'isexpand'` exists only as a proxy argument to that function.
   It does nothing on its own and amounts to misuse of a new option.

The following Vim script function can be used to implement the same
functionality:

```vim
  func CompleteMatch(triggers, sep=',')
    let line = getline('.')->strpart(0, col('.') - 1)
    let result = []
    for trig in split(a:triggers, a:sep)
      let idx = strridx(line, trig)
      if l:idx >= 0
        call add(result, [idx + 1, trig])
      endif
    endfor
    return result
  endfunc
```

related: vim/vim#16716
fixes: vim/vim#18563
closes: vim/vim#18790

cbcbff8712

Co-authored-by: Girish Palya <girishji@gmail.com>

runtime/lua/vim/_meta/vimfn.lua

@@ -1127,53 +1127,6 @@ function vim.fn.complete_check() end
 --- @return table
 function vim.fn.complete_info(what) end
 
---- Searches backward from the given position and returns a List
---- of matches according to the 'isexpand' option.  When no
---- arguments are provided, uses the current cursor position.
----
---- 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.
----
---- Examples: >vim
----   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>
---- <
----
---- @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.