Allow message handlers to be registered dynamically

[sammacbeth]

Description:

Adds an explicit default export from message-handlers.js that contains the default set of message handlers. We then also export a function that allows new handlers to be registered.

This will enable us to move handlers out of the central message handler file, and register handlers per-feature.

[kzar]

Looks good, splitting the message handlers out of this file is something I hoped we'd do and this approach seems pragmatic.

Perhaps add a comment at the top of message-handlers.js (or above the list of default handlers) to advise folks to use registerMessageHandler() in their feature's code instead though. (And maybe to explain that we're migrating away from these default handlers all living in this file.)