Add a `crate-dep-graph` script, remove a few unnecessary dependencies (#3103)

This was motivated by me trying to decide which crate I should put a
`NotificationStore` in.

Run `script/crate-dep-graph` to generate an SVG showing the dependency
graph of our `crates` folder, and open it in a web browser.

After running this command, I noticed a couple of dependencies that
didn't make sense and were easy to remove.

Current dependency graph:

![Screen Shot 2023-10-06 at 1 15 42
PM](https://github.com/zed-industries/zed/assets/326587/b5008235-498a-4562-a826-cc923898c052)