Replace `create_chat_func` with `client` in `querychat_init()`

[gadenbuie]

I'd like to deprecate the create_chat_func in favor of a client argument which could be a function, as in create_chat_func, but could also be an ellmer::Chat object.

This is feasible because we only need to set the system prompt

querychat/pkg-r/R/querychat.R

Line 226 in e2ced68

chat <- create_chat_func(system_prompt = system_prompt)

which is possible using the $set_system_prompt() method.

This does introduce a few new issues:

1. If client is a Chat object, we'll call $clone() to create a new chat.
2. Should we remove turns from the client?
   • I think yes: it maintains current behavior.)
3. Should we remove additional tools from the client?
   • I think maybe not, that could be an interesting way to extend querychat.

[gadenbuie]

It might be useful to introduce an option, e.g. querychat.client, that can be set in the user's .Rprofile to pick a default model for querychat