Implement `wireframeFill` from `BENTLEY_materials_planar_fill`

The [`BENTLEY_materials_planar_fill`](https://github.com/CesiumGS/glTF/tree/vendor-extensions/extensions/2.0/Vendor/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.

Start with processPlanarFill in packages/engine/Source/Scene/Model/MaterialPipelineStage.js, then trace loadPlanarFill in GltfLoader.js and PlanarFill.wireframeFill in ModelComponents.js. Read the BENTLEY_materials_planar_fill reference and the wireframe view work from #13192. Done means the 0, 1, and 2 values control planar fill visibility correctly in wireframe mode.

Written by the indexing model from the issue text.