vim-patch:8.2.2344: using inclusive index for slice is not always desired

Problem:    Using inclusive index for slice is not always desired.
Solution:   Add the slice() method, which has an exclusive index. (closes
            vim/vim#7408)

6601b62943

Cherry-pick a line in docs added later.

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/generators/gen_eval.lua

@@ -17,6 +17,7 @@ hashpipe:write([[
 #include "nvim/cmdexpand.h"
 #include "nvim/cmdhist.h"
 #include "nvim/digraph.h"
+#include "nvim/eval.h"
 #include "nvim/eval/buffer.h"
 #include "nvim/eval/funcs.h"
 #include "nvim/eval/typval.h"