Expose a `destroy` method on Webamp

[Here is a good example](https://github.com/sanity-io/syntax-studio/commit/3d09b263cccacca3aa83cc70c07b2e384025c4a2#diff-fd74a92158d14dfed984441f29a83a37R45) of someone trying to clean up after Webamp. It's not pretty right now, and I'm sure we don't do a good job of cleaning up subscriptions.

I'm not 100% sure how to go about this, since it's going to be very difficult to be fully confident that we've cleaned up everything. Should we ship _something_ and then iteratively ensure that it actually does what it says?