fix: resolve PromiseProxy context memory leak

[edusperoni]

This will cleanup references to resolve and reject after they're called once.

I'm not sure if this will solve the issue for never resolving promises, but in my tests it resolved the issues in @nativescript/core where ImageSource would leak after loading an image from a url.

To go deeper into the issue, this happened because the promise would keep a reference to PromiseProxy's resolve/reject functions, which started to become a problem when this function would then add a native callback, which would keep a reference to the full context, keeping the resolve/reject always in memory (and usually also the promise itself).

Ultimately this created a circular reference with the reactions_or_result GC root that didn't go away until we removed the reference to the original resove/reject functions.

Having trouble following the GC logic? Me too.

Might fix #100. Although I can't reproduce it with the repro code in that issue