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)
• https://github.com/jcheng5/shinychat/pull/5/commits/d38e5e2de648cc8dcdd4bd267d74df7a1445e9b0 (which pulls in https://github.com/posit-dev/py-shiny/commit/e8d1e2c357fec0798c7559a7c14dbd8aba5335c0) 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