tabpage: track last-used tabpage #11626

In a multi-window scenario, it is possible to return focus to the last
accessed window via n_CTRL-W_p.  However, in the case of a multi-tab
scenario, there was previously no way to return focus to the last
accessed *tab*.  Here, that ability is added via n_g<tab>.

Additionally, the index of the previous tab is exposed via
tabpagenr('#'), mirroring the existing functionality of winnr('#').

src/nvim/eval.c

@@ -18172,6 +18172,10 @@ static void f_tabpagenr(typval_T *argvars, typval_T *rettv, FunPtr fptr)
     if (arg != NULL) {
       if (strcmp(arg, "$") == 0) {
         nr = tabpage_index(NULL) - 1;
+      } else if (strcmp(arg, "#") == 0) {
+        nr = valid_tabpage(lastused_tabpage)
+             ? tabpage_index(lastused_tabpage)
+             : nr;
       } else {
         EMSG2(_(e_invexpr2), arg);
       }