Implement VoidCoder

[danielbate]

In sway, 2 types produce a void () value. These are Option::None() and native enums. Both encode to just the case bytes, meaning the value itself is empty.

Internally this is picked up by the TupleCoder due to the bracket regex match, and as it has no children it correctly encodes to empty bytes. However to anyone trying to build this out themselves without using the AbiCoder would have to do this:

const nativeCoder = new EnumCoder('Native', {
    One: new TupleCoder([]),
    Two: new TupleCoder([]),
    Three: new TupleCoder([]),
});

This seems pretty hacky and not immediately obvious to anyone attempting to debug option and native enum problems. We should provide a VoidCoder that does an exact match on the void type for this scenario to provide a better experience for working with void values.

This is a feature but more an implementation detail of our internal encoding, as the VoidCoder itself should not be exported from @fuel-ts/abi-coder.