[BD-3584] fiojs next generation

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This starts the discussion on what the next gen fiojs should look like. Things to consider:

We want to move to a more “modular” architecture for fiojs where there is a “fio addin” on top of a core library.
Next generation eosio sdk will likely not be based on eosjs, but will instead likely be based on the Greymass eosio-core library.
So, first step for next gen fiojs would be to build a new library on top of eosio-core.