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/theme_importer/src/vscode/converter.rs

@@ -167,6 +167,7 @@ impl VsCodeThemeConverter {
             scrollbar_thumb_border: vscode_scrollbar_slider_background.clone(),
             scrollbar_track_background: vscode_editor_background.clone(),
             scrollbar_track_border: vscode_colors.editor_overview_ruler.border.clone(),
+            pane_group_border: vscode_colors.editor_group.border.clone(),
             editor_foreground: vscode_editor_foreground
                 .clone()
                 .or(vscode_token_colors_foreground.clone()),