themes: Add pane_group.border color (#9986)

This PR adds the `pane_group.border` theme attribute that defines the
color of the borders between pane groups.

- Defaults to the `border` color, so nothing changes in the existing
themes.
- VSCode theme converter takes it from the `editorGroup.border`.

The borders marked by red are affected:

<img width="878" alt="pane_group_borders"
src="https://github.com/zed-industries/zed/assets/2101250/54b9fd39-b3e1-4898-a047-ee0b6ec953ed">

Release Notes:

- Added `pane_group.border` to the theme for modifying the border color
for panes within a pane group.

Related Issues:

- First discussed in
https://github.com/zed-industries/zed/pull/9754#issuecomment-2026497213

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/workspace/src/pane_group.rs

@@ -914,7 +914,7 @@ mod element {
                     cx.set_cursor_style(cursor_style, &handle.hitbox);
                     cx.paint_quad(gpui::fill(
                         handle.divider_bounds,
-                        cx.theme().colors().border,
+                        cx.theme().colors().pane_group_border,
                     ));
 
                     cx.on_mouse_event({