[webgpu] Add support for Q2 in matmulnbits (#25763)
### Description
This change adds support for Q2 quantized matmulnbits, in webgpu.
### Motivation and Context
An alternate way to support bitnets is through adding support for lower
bits in matmulnbits, this reuses our shaders and is more maintainable
than a separate op. The model size grows a bit however for a 2B
parameter model using 1.58bpw vs 2bpw the size difference is just 100MB.
The simpler dequantization also improves perf, on an Intel XE matmul
looks to be 20% faster using q2 weights vs q4 weights for the same
matrix dimensions.
Q2 version of the bitnet model is here
https://huggingface.co/sushraja/bitnet-b1.58-2B-4T-fp16-onnx/tree/main/bitnet_q2
