Add support for weak references

[devongovett]

This adds support for a WeakReference type, which is similar to Reference but does not increment the napi reference count. This is useful in cases where you need to avoid a cycle, e.g. a parent object with a reference to a child, which also has a reference back to the parent. Just using Reference for this results in the objects never being garbage collected.

A WeakReference can be created from a Reference using the downgrade method, and either returned from an napi function by cloning, or converted back to a strong reference with the upgrade method. This API is inspired by the Rust Rc and Weak types.