Rewrite: Support for custom JSON encoder

[scragly]

In line with PR #65 and the recommendation over at #68 (comment) by @Gelbpunkt, we'll try supporting custom JSON encoders in a more tightly integrated / cleaner manner in the new rewrite.

Possible alternative serialisers would be orjson and ujson.

Considerations of note

Due to orjson being a BSON serialiser, and Lavalink not accepting bytes, we should ensure we don't just focus on a basic swap of dumps, as we'll need to convert to unicode when testing orjson but shouldn't attempt to encode where unnecessary.