Fix dynamic UI

[cpsievert]

Closes #4

Note that:

• 3fe1f93 is mainly for sanity reasons -- to make sure the way in which chat.js gets executed is the same as py-shiny
• 8e5671c is the heart of the fix (wait until flush to send messages)
• jcheng5/shinychat@d38e5e2 (which pulls in posit-dev/py-shiny@e8d1e2c) makes it so chat.js immediately subscribes to the custom messages. For some reason, I was noticing that (sometimes?!) starting messages wouldn't get handled without this, since the message would get processed before the handler was registered. The only reason that $(function() {}) was there in the 1st place was that I was following what data-frame does, but it seems unnecessary/better to subscribe immediately