Hook up example events -> dashboard communication

[gnidan]

This PR does a few things:

• Add a new "debug" message type, intended for logging in the browser console. This message type behaves like "provider" messages, in that it waits for a response from a subscriber.
• Handle this new message type in the frontend by logging and then issuing a blank response to clear it from the message-bus requests queue
• Initialize a quick DashboardMessageBusClient for use in @truffle/events. Define a sendAndAwait method to behave similarly to the @truffle/dashboard-message-bus util function
• Use this client to send an example debug message to the browser upon "compile:start"

[cds-amal]

Nice and quick. I like things that unblock.

[gnidan]

Thanks for the fix @benjamincburns... your changes look good to me, so I think we can merge this!