patch 9.1.1422: scheduling of complete function can be improved

patch 9.1.1422: scheduling of complete function can be improved

Problem:  scheduling of complete function can be improved
Solution: call user completion functions earlier when just determining
          the insertion column (Girish Palya)

This change improves the scheduling behavior of async user-defined
completion functions (such as `F{func}`, `F`, or `'o'` values in the
`'complete'` option), particularly benefiting LSP clients.

Currently, these user functions are invoked twice:

1. First with `findstart = 1` to determine the completion start
   position.
2. Then with `findstart = 0` to retrieve the actual matches.

Previously, both calls were executed back-to-back. With this change, the
first call (`findstart = 1`) is performed earlier—before any matches are
gathered from other sources.

This adjustment gives event-driven completion sources (e.g., LSP
clients) more time to send their requests while Vim concurrently
collects matches from other sources like the current buffer.

Not sure about the real-world performance gains, but this approach
should, in theory, improve responsiveness and reduce latency for
asynchronous completions.

To test, try using yegappan LSP client:

```vim
set cpt+=o^10
autocmd VimEnter * g:LspOptionsSet({ autoComplete: false, omniComplete: true })
```

If you prefer to use 'native' auto-completion (without plugins), try the
following configuration:

```vim
set cot=menuone,popup,noselect,nearest
autocmd TextChangedI * InsComplete()
def InsComplete()
  if getcharstr(1) == '' && getline('.')->strpart(0, col('.') - 1) =~ '\k$'
    SkipTextChangedI()
    feedkeys("\<c-n>", "n")
  endif
enddef
inoremap <silent> <c-e> <c-r>=<SID>SkipTextChangedI()<cr><c-e>
inoremap <silent> <c-y> <c-r>=<SID>SkipTextChangedI()<cr><c-y>
def SkipTextChangedI(): string
  set eventignore+=TextChangedI
  timer_start(1, (_) => {
    set eventignore-=TextChangedI
  })
  return ''
enddef
inoremap <silent><expr> <tab> pumvisible() ? "\<c-n>" : "\<tab>"
inoremap <silent><expr> <s-tab> pumvisible() ? "\<c-p>" : "\<s-tab>"
```

closes: #17396

Signed-off-by: Girish Palya <girishji@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>