This PR adds the `gpt-4-1106-preview` model. It required forking two
repos from different organizations, as one is a submodule of another.

Update 1:

~Currently, I'm just temporarily pointing towards a personal fork of the
repo, but once [this
PR](https://github.com/zurawiki/tiktoken-rs/pull/49) lands, we can
switch back to using the official crate.~

Update 2:

~PR was merged, we are now pointing to a specific commit on the original
official repo - may want to bump it to an official version once a new
one has been released.~

Update 3:

Author published a [new
version](https://crates.io/crates/tiktoken-rs/0.5.7). Now, we are back
to relying on the official crate.

<img width="1118" alt="SCR-20231114-ibgs"
src="https://github.com/zed-industries/zed/assets/19867440/cc2657b6-98f9-413f-9a0c-f337636f367c">

Release Notes:

- Added support for the
[`gpt-4-1106-preview`](https://openai.com/blog/new-models-and-developer-products-announced-at-devday)
model in the assistant panel.
- Updated the `assistant.default_open_ai_model` setting to default to
`gpt-4-1106-preview`. To override the default, use the following snippet
to your `settings.json` file:

```rs
"assistant": {
    // 1. "gpt-3.5-turbo-0613""
    // 2. "gpt-4-0613""
    // 3. "gpt-4-1106-preview"
    "default_open_ai_model": "<Your choice of model here>"
  },
```