improvement: add `disposeOld` parameter to `setContext`

[WofWca]

This makes it harder to introduce a resource leak.

This is especially useful when you pass native AudioContext to setContext. Consider these examples, where it doesn't look like there are any memory leaks, but actually there are:

• Repeatedly calling setContext with the same native AudioContext creates undisposed unused Tone Contexts.
• Repeatedly calling

  myContext = new AudioContext();
  Tone.setContext(myContext);
  myContext.close();

  does the same.

It looks to me like a real solution would be to make setContext only accept BaseContext, shifting the burden of creating and disposing Tone Context/OfflineContext to the user, but that would be breaking. The disposeOld parameter could perhaps also be preserved.

I only ran test:html.