Fullspec msgpack decode

Current "From MessagePack" works only for limited cases. For example, it does not support integer map keys, which are valid in msgpack.

Generally, not all valid msgpack sequences can be converted to json.

**Describe the solution you'd like**
Decode MsgPack to some readable text representation (make json as an option).

Start by locating CyberChef's current “From MessagePack” operation and its tests, then inspect how decoded values are converted to JSON, especially integer map keys. Done means valid MessagePack sequences can be shown in a readable text representation, with JSON conversion available as an option.

Written by the indexing model from the issue text.