WebGPUBackend: Allow Storage Buffer on Index Attribute

[RenaudRohlinger]

Description
Allow use of Storage Buffer on Index Attribute in WebGPU. This can be useful for Marching Cubes or Tessellation for examples.

This contribution is funded by Utsubo

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	337.69 78.76	337.69 78.76	+0 B +0 B
WebGPU	557.08 154.25	557.18 154.29	+103 B +36 B
WebGPU Nodes	556 154.04	556.1 154.07	+103 B +35 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	468.91 113.45	468.91 113.45	+0 B +0 B
WebGPU	632.74 171.27	632.84 171.31	+103 B +39 B
WebGPU Nodes	587.87 160.63	587.98 160.66	+103 B +37 B