Base Unit class implements JsonSerializable now

[boris-glumpler]

I'm using this library in a Laravel project where I cast various values to units to be able to easily convert them. The admin area is built with Filament/Livewire. Livewire validates the current request data by creating a JSON fingerprint from it. Somehow a unit is occasionally encoded as {} or as [] in this fingerprint, causing a mismatch and an error. My changes fixes this.

My particular use case here sounds a bit weird tbh, but being able to encode a unit as JSON could be quite useful in other areas, like API responses for example. Anyways, I'd appreciate it if you could merge these changes. Cheers!