feat: Cache toObject(...) using jsi::WeakObject and correctly set externalExternalMemoryPressure

[mrousavy]

This PR now improves performance of passing HybridObjects to JS.

1. jsi::Object instances of a given HybridObject are now cached using WeakObject, and can be re-used in subsequent calls.
   Subsequent calls use the cached object and return it to JS, which avoids going the long prototype assignment route.
2. Additionally, setExternalMemoryPressure can now finally be used properly, since we know that we only have one jsi::Object instance of the same HybridObject in memory, so it will no longer track memory for the same object twice/thrice/...! 🥳
3. And lastly; obj.foo == obj.foo now works, since we actually return the same jsi::Object! It's cached in a weakref, so that finally works. cc @wcandillon

[mrousavy]

cc @hannojg because setExternalMemoryPressure now works
cc @wcandillon because obj.foo == obj.foo now works

[wcandillon]

nice :)

[mrousavy]

released in 0.2.0