Refactor state management to decouple from Howler

[E-Kuerschner]

To future proof the library while preserving the API, we should introduce a strict abstraction layer and a concrete implementation layer for the state management. This could help if we ever need to swap Howler out with a different sound engine or a custom implementation.

From comments in code:
// TODO: the main state reducer should be decoupled from Howler
// to accomplish this, each action should describe the type of change using an abstraction rather than passing in the howl