Separate `red_knot` into CLI and `red_knot_workspace` crates (#12623)
## Summary
This PR separates the current `red_knot` crate into two crates:
1. `red_knot` - This will be similar to the `ruff` crate, it'll act as
the CLI crate
2. `red_knot_workspace` - This includes everything except for the CLI
functionality from the existing `red_knot` crate
Note that the code related to the file watcher is in
`red_knot_workspace` for now but might be required to extract it out in
the future.
The main motivation for this change is so that we can have a `red_knot
server` command. This makes it easier to test the server out without
making any changes in the VS Code extension. All we need is to specify
the `red_knot` executable path in `ruff.path` extension setting.
## Test Plan
- `cargo build`
- `cargo clippy --workspace --all-targets --all-features`
- `cargo shear --fix`