Wasm gemm interface update

[abhi-agg]

Description

This PR improves on the interface introduced in #49

List of changes:

• Added separate scale factor and zero point for both A and B in int8PrepareBias and int8MultiplyAndAddBias interface instead of pre-computing the unquantization value and passing it as a single parameter
• An additional unquantization multiplier parameter in int8MultiplyAndAddBias
• Updated the corresponding fallback implementation

Added dependencies: none

Checklist

• I have tested the code manually
• I have run regression tests
• I have read and followed CONTRIBUTING.md
• I have updated CHANGELOG.md

[abhi-agg]

@kpu I made these changes as per our discussion in the last plenary meeting 👍🏾

[kpu]

I can't think of a use of the unquant_multiplier in prepareBias since it's just supposed to make the bias term ready. You seem you agree with that by not using it. Suggest removing it as an argument from the prepareBias call.
Keep the factor in the multiply though.
Then otherwise good to go.

[abhi-agg]

@kpu I removed unquant_multiplier from prepareBias API as per your recommendation 👍🏾