feat: chat_restore() replaces chat_enable_bookmarking()

[gadenbuie]

Replaces chat_enable_bookmarking() with chat_restore(), which now also restores the message history either from the bookmark or from the messages on client when passed in.

@schloerke and I paired through getting chat_enable_bookmarking() to work inside modules. Along the way, we realized that it'd be best for this function to also handle setting UI state from the message history attached to the chat client, in particular because we only want to perform that restore if we're not restoring from a bookmark.

This change meant that chat_enable_bookmarking() was not quite accurately named. Also, because chat_enable_bookmarking() still requires enableBookmarking to be turned on in the Shiny app, chat_restore() feels a bit more generally appropriate.

We also now turn on bookmarking in chat_app() and we also avoid the issues described in #71.

Finally, chat_mod_server() gains the bookmark_on_* arguments of chat_restore() and does the restoration automatically. And we deprecate client in chat_mod_ui() because this is now handled entirely on the server.

Implementation note: this PR includes a contents_shinychat() placeholder that will be replaced with contents_shinychat() in #52.

[cpsievert]

Will this also clear starting messages (i.e., chat_ui(messages = list(...)))?

[gadenbuie]

Good point and yeah, that's true. I've updated the docs of chat_mod_ui() to clarify that messages only applies when the client doesn't have any history. And left a similar note in the chat_restore() docs.