fix: memory leak in local process extension host

[SimonSiefke]

Fixes a memory leak in local process extension host by setting the port message listener to null on dispose

portPromise.then((port) => {
	this._register(toDisposable(() => {
		port.close();
		port.onmessage = null; // here
	}));
	clearTimeout(handle);

});

Also changes this._toDispose.add to using this._register which overall seems a bit simpler.

Before & After Comparison

Barely noticable, but one can see at the bottom there is one less leak.

The chart shows restarting extensions 97 times, before and after.

[alexdima]

Thank you!