Remove passing load fallback to itself as an argument

[saitonakamura]

And updates types accordingly. Looking at the docs, there's no 3rd argument to the fallback and it doesn't really make sense to pass fallback itself.

https://nodejs.org/api/esm.html#loadurl-context-nextload

I guess its presense doesn't really hurt but it's redundant so I took the liberty of fixing it. Let me know what you think if you have any concerns.