vim-patch:8.2.1401: cannot jump to the last used tabpage

Problem:    Cannot jump to the last used tabpage.
Solution:   Add g<Tab> and tabpagnr('#'). (Yegappan Lakshmanan, closes vim/vim#6661,
            neovim #11626)
62a232506d

Nvim implemented this feature before Vim, but Vim made some useful changes (e.g:
beeping on failure). Port the changes to closer match Vim (also makes porting
future patches easier).

Also note that because CHECK_CMDWIN was added to goto_tabpage_tp, there is no
need to do the extra work with tabpage_index and goto_tabpage inside
goto_tabpage_lastused to fix cmdwin issues any more (#11692).
Note that while goto_tabpage_tp doesn't check for textlock like goto_tabpage
does, it shouldn't matter as it is already checked for earlier.

Add tags for <C-Tab> to tabpage.txt, and refer to <C-Tab> over CTRL-Tab to be
consistent with other docs like the patch.
Remove mention of "previous tabpage" (it can be confused with the tabpage to the
left, e.g: `:tabprevious`).
Similarly, don't rename old_curtab to last_tab in enter_tabpage (it might be
confused with the right-most tabpage, e.g: `:tablast`).

Cherry-pick Test_tabpage change from v8.2.0634.
92b83ccfda

src/nvim/eval/funcs.c

@@ -11213,9 +11213,7 @@ static void f_tabpagenr(typval_T *argvars, typval_T *rettv, FunPtr fptr)
       if (strcmp(arg, "$") == 0) {
         nr = tabpage_index(NULL) - 1;
       } else if (strcmp(arg, "#") == 0) {
-        nr = valid_tabpage(lastused_tabpage)
-             ? tabpage_index(lastused_tabpage)
-             : nr;
+        nr = valid_tabpage(lastused_tabpage) ? tabpage_index(lastused_tabpage) : 0;
       } else {
         semsg(_(e_invexpr2), arg);
       }