Show triangle count in 3D Tiles inspector

[lilleyse]

@rahwang more enhancements for the 3D Tiles inspector, probably worth doing before #5156.

For this we want to see the total number of triangles that are rendered. This will operate similarly to the memory usage statistics added in #5050.

• For each *3DTileContent file add a trianglesLength getter, which can be modeled after vertexMemorySizeInBytes. It is only relevant for Batched3DModel3DTileContent and Instanced3DModel3DTileContent right now, but the others should include it.
• Add a getter to Model, also modeled after vertexMemorySizeInBytes. The triangle count can be added when the DrawCommands are created, since the count variable is accessible there.
• Update the statistics in Cesium3DTileset, like how it's done for numberOfPointsSelected. numberOfTrianglesSelected is an ok name for now.
• Add to the UI in Cesium3DTilesInspectorViewModel.
• Add a test for Model, Batched3DModel3DTileContent, Instanced3DModel3DTileContent that check the triangle length are correct. These can be similar to the gets memory usage tests where it loads a model or tileset and then gets the trianglesLength property and checks that it is correct.
• Test in Cesium3DTileset. You may be able to just modify all the tests that call checkPointAndFeatureCounts and supply the expected triangle length.

[lilleyse]

Fixed in #5183