Estimate value sizes for Deno KV

[kitsonk]

Related #18415, #12379

Deno KV currently has a value limit of 64KiB. While determining the size of things like strings and array buffers are straight forward, serialized objects are not straight forward. Especially things like Map() and Set() which may have deeply nested complex objects. Attempting the store the value and managing exceptions doesn't feel like a logical way to manage it. A user would want to know if a potential value is too big to store, potentially breaking it down and storing it as sub keys of an additional key. Like for example if an instance of a Map() is too big, recursively iterate over the keys, and store each key individually once it was "small enough".

A straight forward API that allows sizing of a value to be stored. While Deno.core.serialize() is available, it rightfully so isn't a public API, and even then it would be wasteful to allocate the ArrayBuffer in memory, only to get its size and discard it, likely to store it in the database.