impossible to use 0 as extension type ?

First of all, thanks for making this great package!

Docs state that `extension type code (should be a number from 1-100, reserving negatives for MessagePack, 101-127 for msgpackr)`.

But in some case, we might need to use 0 as extension type, got error

```
Uncaught TypeError: currentExtensions[type] is not a function
```

Just wanna make sure this is impossible, right ?