Implement FromJSValConvertible for *mut JSObject

[KiChjang]

r? @Ms2ger or @nox

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This change is 

[Ms2ger]

Doesn't compile

[KiChjang]

Uhh right, stupid me.

[Ms2ger]

These implementations crash if handed the wrong type of value. Is that intentional?

[KiChjang]

I essentially copied the implementation from JSVal. Shall I instead check that it is an object first, and if not, return an Err(()) or a ConversionResult::Failure?

[Ms2ger]

Let's take a step back. Why did you make this PR?

[KiChjang]

MessagePort contains a WebIDL method that looks like this:

  void postMessage(any message, optional sequence<object> transfer = []);

And so CodegenRust.py is trying to convert a JS HandleValue to a JS object, which currently doesn't exist in rust-mozjs bindings.

[jdm]

Given that this is only needed because of codegen for a feature we don't support yet (transferrable objects), I propose we use optional any transfer and close this PR.