Expose Proxy builtin as a public API

[mikroskeem]

As per discussion in Discord:

[8:23 PM] Jedel: I see. We currently haven't exposed the Proxy builtin as a public API. However, we're implementing safe wrappers for every object type to make them easier to use in use cases like yours
[8:25 PM] Jedel: If you want, you can open an issue in our repo and we'll work on implementing a safe wrapper for Proxy
[8:30 PM] mikroskeem: hm, I'm not sure if that is the best approach to simply create a rust wrapper for Proxy object
[8:30 PM] mikroskeem: but then again I'm not familiar with Boa internals at all
[8:30 PM] Jedel: The difference would be that a wrapper for Proxy would make it possible to pass native functions as proxy handlers
[8:31 PM] Jedel: whereas a Proxy object inside JS uses JS functions

My use-case would be defining an proxy in native code, e.g to emulate process.env[key] for code I cannot control.

Simply put, instead of evaluating this before running vendor code:

globalThis.process = {
  env: new Proxy({}, {
    get: (_self, key) => myBoaNativeFuncToGetEnv(key),
    //set: (_self, key, value) => { throw new Error("Not supported"); },
  }),
};

could be replaced with 100% Rust counterpart.