improvement: default disposeOld of setContext to true

[WofWca]

BREAKING CHANGE: if the user doesn't want the old context to get disposed, he must explicitly pass false

Depends on #880

Marked as WIP because

• it's breaking.
• There might be better ways, like making the parameter required instead, or, as said in improvement: add disposeOld parameter to setContext #880, making setContext not accept AnyAudioContext.
• Not sure whether it's usage accross the code base is correct (e.g.

  Tone.js/Tone/core/context/Offline.ts

  Line 58 in 599e61f

  setContext(originalContext);

  – should the OfflineContext be disposed?)

[tambien]

Merging #880, but i think i won't merge this one for now. I think this feature would make more sense if it only disposed of any automatically created AudioContext, but i think in terms of the ownership of resources, it makes more sense for whoever created the AudioContext to dispose of that AudioContext instead of it being disposed of automatically.