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

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