Add support for the "loudness" block and "loudness greater than" trigger

[adroitwhiz]

Based on @apple502j's work, with some refactoring and rebasing to work with the new edge-activated trigger code and also clean up the code a bit.

Resolves #122 and closes #60.

[PullJosh]

Any idea why this.loudness is always returning 0? (Except before connecting, when it returns -1.)

The variable watcher is displaying this.loudness from within the stage, and the cat is saying this.loudness from within the sprite.

[PullJosh]

I am confused... I changed nothing, but an hour later, this.loudness is working perfectly. 🤔

Edit: Aaaaaannnd now it's broken again.

[PullJosh]

Oh, I see. Chrome has been giving me the following warning for a long time now (since long before this PR):

The AudioContext was not allowed to start. It must be resumed (or created) after a user gesture on the page. https://goo.gl/7K7WLu

I assumed this was for playing sounds only, but it appears that the microphone access also relies on Sound.audioContext, and that audioContext is suspended (and thus the mic gets no data--all zeroes).

[PullJosh]

Behavior looks good now 👍

(@towerofnix said they might be away, so I removed them as a reviewer.)