vim-patch:9.1.1797: completion: autocompletion can be improved

Problem:  completion: autocompletion can be improved
Solution: Add support for "longest" and "preinsert" in 'autocomplete';
          add preinserted() (Girish Palya)

* Add support for "longest" in 'completeopt' when 'autocomplete'
  is enabled. (Note: the cursor position does not change automatically
  when 'autocomplete' is enabled.)
* Add support for "preinsert" when 'autocomplete' is enabled. Ensure
  "preinsert" works the same with and without 'autocomplete'
* introduce the preinserted() Vim script function, useful for defining
  custom key mappings.

fixes: vim/vim#18314
closes: vim/vim#18387

c05335082a

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

src/nvim/eval.lua

@@ -8024,6 +8024,18 @@ M.funcs = {
     returns = 'number',
     signature = 'pow({x}, {y})',
   },
+  preinserted = {
+    desc = [=[
+      Returns non-zero if text has been inserted after the cursor
+      because "preinsert" is present in 'completeopt', or if
+      "longest" is present in 'completeopt' while 'autocomplete'
+      is enabled.  Otherwise returns zero.
+    ]=],
+    name = 'preinserted',
+    params = {},
+    returns = 'number',
+    signature = 'preinserted()',
+  },
   prevnonblank = {
     args = 1,
     base = 1,