Break ground on `assistant2` (#21109)

This PR breaks ground on a new `assistant2` crate.

In order to see this new version of the assistant, both of the following
must be true:
1. The `assistant2` feature flag is enabled for your user
   - It is **not** currently enabled for all staff.
2. You are running a development build of Zed

The intent here is to enable the folks working on `assistant2` to
incrementally land work onto `main` without breaking use of the current
Assistant for anyone.

<img width="1136" alt="Screenshot 2024-11-23 at 10 46 08 AM"
src="https://github.com/user-attachments/assets/5723a13f-5be1-4486-9460-ead7329ba78e">

Release Notes:

- N/A