perf(events): store autocommands in flat vectors (#23256)

Instead of nested linked lists, store autocommands in a flat, contiguous
kvec_t, with one kvec_t per event type. Previously patterns were stored
in each node of the outer linked list, so they can be matched only once
on repeating patterns. They are now reference counted and referenced in
each autocommand, and matching is skipped if the pattern repeats. Speeds
up creation and deletion, execution is not affected.

Co-authored-by: ii14 <ii14@users.noreply.github.com>

src/nvim/ex_docmd.c

@@ -1885,8 +1885,7 @@ static char *do_one_cmd(char **cmdlinep, int flags, cstack_T *cstack, LineGetter
       // avoid that a function call in 'statusline' does this
       && !getline_equal(fgetline, cookie, get_func_line)
       // avoid that an autocommand, e.g. QuitPre, does this
-      && !getline_equal(fgetline, cookie,
-                        getnextac)) {
+      && !getline_equal(fgetline, cookie, getnextac)) {
     quitmore--;
   }