[red-knot] IDE crate (#17045)
## Summary
This PR adds a new but so far empty and unused `red_knot_ide` crate.
This new crate's purpose is to implement IDE-specific functionality,
such as go to definition, hover, completion, etc., which are used by
both the LSP and the playground.
The crate itself doesn't depend on `lsptypes`. The idea is that the
facade crates (e.g., `red_knot_server`) convert external to internal
types.
Not only allows this to share the logic between server and playground,
it also ensures that the core functionality is easier to test because it
can be tested without needing a full LSP.
## Test Plan
`cargo build`
