Implement `serde::(De)Serialize` for `keyvalues_parser::Vdf`

Implementing this would be a huge ergonomics win for users, but beyond that it would unlock much better fuzzing potential for `keyvalues-serde`. Right now `keyvalues-serde` currently fuzzes an arbitrarily generated "kitchen sink" style struct that holds a lot of different data types, but that doesn't catch issues like #45 :crying_cat_face:

Having this implemented would give us the arbitrarily dynamic type that would be perfect for fuzzing to find since it's the one type that describes all valid VDF blobs