feat: add support for global statusline

Ref: #9342

Adds the option to have a single global statusline for the current window at the bottom of the screen instead of a statusline at the bottom of every window. Enabled by setting `laststatus = 3`.

Due to the fact that statuslines at the bottom of windows are removed when global statusline is enabled, horizontal separators are used instead to separate horizontal splits. The horizontal separator character is configurable through the`horiz` item in `'fillchars'`. Separator connector characters are also used to connect the horizontal and vertical separators together, which are also configurable through the `horizup`, `horizdown`, `vertleft`, `vertright` and `verthoriz` items in `fillchars`.

The window separators are highlighted using the `WinSeparator` highlight group, which supersedes `VertSplit` and is linked to `VertSplit` by default in order to maintain backwards compatibility.

src/nvim/buffer.c

@@ -5006,8 +5006,8 @@ void ex_buffer_all(exarg_T *eap)
       wpnext = wp->w_next;
       if ((wp->w_buffer->b_nwindows > 1
            || ((cmdmod.split & WSP_VERT)
-               ? wp->w_height + wp->w_status_height < Rows - p_ch
-               - tabline_height()
+               ? wp->w_height + wp->w_hsep_height + wp->w_status_height < Rows - p_ch
+               - tabline_height() - global_stl_height()
                : wp->w_width != Columns)
            || (had_tab > 0 && wp != firstwin))
           && !ONE_WINDOW