TSL: Fix bitcast type resolver

[cmhhelgeson]

Related issue: #31735

Description

Fixes bitcast functionality on WebGPU backend. Bitcast functionality was previously not working since the MathNode could not resolve getNodeType from the type string passed to bitcast.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.91 79.11	338.91 79.11	+0 B +0 B
WebGPU	578.39 159.34	578.96 159.5	+570 B +156 B
WebGPU Nodes	577 159.1	577.57 159.26	+570 B +158 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	470.78 113.9	470.78 113.9	+0 B +0 B
WebGPU	648.93 175.37	649.16 175.44	+232 B +71 B
WebGPU Nodes	603.02 164.5	603.25 164.57	+232 B +69 B

[cmhhelgeson]

Githack below just arbitrarily modifies webgpu_compute_geometry to use bitcasting on instanceIndex, and demonstrates the bitcast working by logging the compute shader that uses it.

https://raw.githack.com/cmhhelgeson/three.js/cmh/fix_bitcast_wgsl_test/examples/webgpu_compute_geometry.html