Add theme color customization for idle sashes

Description
With the introduction of "workbench.experimental.modernUI": true, the split/resize sashes are significantly more prominent in the UI.

Currently, there is no theme color token in workbench.colorCustomizations to modify the background color of sashes when they are in an idle (non-hovered) state.

The API only exposes sash.hoverBorder to color sashes during mouse hover or active dragging. In an idle state, sashes either rely on default container borders or remain hard-coded/unstyled, leaving no mechanism to customize the static background color of the sash handles.

Proposed Solution
Introduce a new color token such as sash.backgroundColor (or equivalent) to the color theme schema to allow customizing the default idle appearance of sashes across the workbench.