Implement `wireframeFill` from `BENTLEY_materials_planar_fill`

[markschlosseratbentley]

The BENTLEY_materials_planar_fill glTF extension (partial reference implementation in #13178) defines a wireframeFill property that controls whether a planar polygon's interior fill is displayed in a wireframe view mode:

Value	Name	Description
0	NONE	The fill is never drawn in wireframe view.
1	ALWAYS	The fill is always drawn in wireframe view.
2	TOGGLE	The fill is drawn only if the user or application has enabled toggleable fill display.

Proposed solution

A proper wireframe view mode has been added to CesiumJS (#13192), so we should implement wireframeFill support (likely in MaterialPipelineStage) to control fill visibility in that mode.

Code referencing this issue:

• packages/engine/Source/Scene/Model/MaterialPipelineStage.js (processPlanarFill)
• packages/engine/Source/Scene/GltfLoader.js (loadPlanarFill)
• packages/engine/Source/Scene/ModelComponents.js (PlanarFill.wireframeFill)

Partially blocks completion of #12890.