vim-patch:9.1.1742: complete: preinsert does not work well with 'autocomplete' (#35692)

Problem:  complete: preinsert does not work well with preinsert
Solution: Make "preinsert" completeopt value work with autocompletion
          (Girish Palya)

This change extends Insert mode autocompletion so that 'preinsert' also
works when 'autocomplete' is enabled.

Try: `:set ac cot=preinsert`

See `:help 'cot'` for more details.

closes: vim/vim#18213

fa6fd41a94

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

src/nvim/highlight.h

@@ -85,6 +85,7 @@ EXTERN const char *hlf_names[] INIT( = {
   [HLF_BFOOTER] = "FloatFooter",
   [HLF_TS] = "StatusLineTerm",
   [HLF_TSNC] = "StatusLineTermNC",
+  [HLF_PRE] = "PreInsert",
 });
 
 EXTERN int highlight_attr[HLF_COUNT];     // Highl. attr for each context.