Import fallback and WASM portable matmul here

[jerinphilip]

There's the following duplication:

• https://github.com/browsermt/bergamot-translator/blob/9eb243725bdfa8fef32225c46d4673af6f144251/wasm/import-gemm-module.js
• https://github.com/browsermt/marian-dev/blob/08b1544636fe13eaf1fbacb17c6fb050abfb8d42/wasm/import-gemm-module.js

The above need to be resolved from here, is what I understand from browsermt/marian-dev#56 (comment):

Maybe or maybe not. The ideal location for fallback implementation is in separate wasm module and in a completely new project, you have to start and maintain as dependency. Keeping fallback logic in the Marian for first iteration will also save you some time, you would spend on dealing of wasm memory instantiation when multiple wasm modules are used.

Just adding this issue and linking to places, for now.