Remove Terminal.destroy

[Tyriar]

Let's standardize on dispose #1370

[mofux]

The disposable pattern got a lot of traction recently with OSS like vscode and atom using this concept to track and release dependant resources. Basically, every object that has a dispose method is seen as a disposable. The creator of a disposable (usually also a disposable) can track it, and then dispose its dependencies once it gets disposed itself.

I would also like to change our own EventEmitter implementation to fully use this pattern, so destroy becomes dispose, and the on listener returns a disposable.

[Tyriar]

👍 started in #1372