patch 9.1.1742: complete: preinsert does not work well with preinsert

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: #18213

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

src/vim.h

@@ -1587,6 +1587,7 @@ typedef enum
     , HLF_TPL	    // tabpanel
     , HLF_TPLS	    // tabpanel selected
     , HLF_TPLF	    // tabpanel filler
+    , HLF_PRI	    // "preinsert" in 'completeopt'
     , HLF_COUNT	    // MUST be the last one
 } hlf_T;
 
@@ -1599,7 +1600,7 @@ typedef enum
 		  '+', '=', 'k', '<','[', ']', '{', '}', 'x', 'X', \
 		  '*', '#', '_', '!', '.', 'o', 'q', \
 		  'z', 'Z', 'g', \
-		  '%', '^', '&' }
+		  '%', '^', '&', 'I'}
 
 /*
  * Values for behaviour in spell_move_to