feat(core/serde): add NumericValue container

[kuhe]

• creates the core/serde submodule, which will later be expanded when much of serde moves from codegen to JS runtime
• adds a NumericValue container (unused so far) for holding BigDecimal data without deciding on a numeric library on behalf of the user

This is not currently in use, but will facilitate future work on serde. It will also unify behavior related to too-large numerics. Currently there is inconsistent behavior among protocols when handling large numbers. Some make best-effort attempts to fit into Number and some throw errors disallowing large numbers. Both of those are not desirable for customers.

We should aim to preserve numeric precision in all cases without coupling to any specific numeric library. A simple string container with a type differentiator field accomplishes that.