Refactor `NbtEncoder`/`NbtDecoder` implementations

[BenWoodworth]

Becoming difficult to maintain, and the two implementations are very different.

If possible, leaning on a more stateless design and leaning on SerialDescriptors would be ideal.

TODO:

• [ ] Re-write with minimal abstractions, so logic is easier to follow
• [ ] Don't use values to determine how to encode. SerialDescriptor should be the source of truth (see: here)
• [ ] Track NBT path more nicely