feat(neon): Implement TryIntoJs/TryFromJs for either::Either

[kjvalencik]

This implements the extract traits for either::Either. I chose not to feature flag it because either is a very small crate with zero dependencies.

The Ok implementation paths were very clear. However, there isn't an obviously correct implementation for a failed TryFromJs.

We have two errors, both L::Error and R::Error. This PR does the following:

• Creates a new Error that prints the type name of L and R saying that they failed
• Attaches left and right keys to the error object with their TryIntoJs implementation

Another option would be to return only one of the errors. Unfortunately, we can't attach one error to the other because it might not be an Object.

What's the best thing to do for this?