(WIP) Sponge support

[SamB440]

Adds Sponge support. A lot is copied from the Spigot module as they are fundamentally similar and via runs on Sponge too.

The only conversion util I made is itemstack conversion. Most things can be done via Sponge API or VanillaGradle anyway.

~~Also fixed use of deprecated java.sourceCompatibility and java.targetCompatibility (need to confirm it compiles to 8 bytecode again).~~

[SamB440]

Update for those unaware: viaversion support doesn't seem to be possible because on Sponge they replace the vanilla handler, instead of adding their own.

[SamB440]

I think I might migrate the current injection stuff to be mixin-based like fabric

[retrooper]

Update for those unaware: viaversion support doesn't seem to be possible because on Sponge they replace the vanilla handler, instead of adding their own.

They have fixed this recently.

[retrooper]

You can mark this as a PR.